MJW

Plugins can now officially write outside the selection, though I believe that's limited to the two new-fangled varieties, not the "classic" plugin we know and love. Fortunately, one of the new style of plugins can do the things the old-style plugins can do. So far though, unless I somehow missed it, plugins still can't change the selection. You can write a modified selection to the clipboard, then load it with Paste Selection, but obviously that's a bit awkward and inflexible.
 
 

If I correctly understand what you're trying to do (which I may not), copy one image to the clipboard, then paste it as a new layer into the other image. Set the Opacity of the top layer to 127. Merge the layers. (The average color will be very slightly off, since 255 isn't evenly divisible by 2.)

That control can be confusing. To rotate in a negative direction around the Y axis, do one of two things:
 
1) Move the second slider to the right all the way to 180. The third slider will now rotate the plane in the negative direction; or,
2) Hold down the Shift key, and move center of the ball control leftward. (This will automatically move the second slider to 180.) Holding down the Shift key confines the direction of the tilt to horizontal or vertical.
 
Because the Rotate/Zoom control is confusing, I almost always first use the ball control to tilt the plane to the general desired position, then tweak it with the sliders. I think the Rotate/Zoom control follows the standard convention for pitch/yaw/roll, or something like that.
 
 
 
 

The data may be incorrect because you're not doing the indexing correctly. Two 8-bit Bitmap files can be functionally identical (i.e., will produce the same image) but completely different internally if the colors in the color table are in different orders. There is no right or wrong order for the color table entries, so to process an 8-bit Bitmap file, you must use the 8-bit values as indices into the color table. The color table (a property called Palette) contains an array of Colors (called Entries). The 8-bit color values are used to index into that array to fetch the ARGB color that corresponds to the value.
 
EDIT: Let me say it in a slightly different way, that may be clearer. If the 8-bit color is, for instance, 20, that doesn't mean it's the 20th level of brightness or anything like that. It means it's the 20th color in the color table. To find out what that color is, you must index into the table.

I think that's slightly incorrect. Left or right clicking on the color wheel selects whichever one, Primary or Secondary, is the current choice in the drop-down box at the top of the Color menu. I believe it would be better if it worked like the Dropper. Or perhaps left clicking should select whichever is the current choice, while right clicking should switch the choice, and then select that color. That seems very practical, but it might be confusing. (I don't think the Dropper could work that way, because then the user would have to see the Color menu to know which was being chosen.)
 
I think the original commenter has a good point. Maybe there could be a setting for whether the Color menu is initially expanded. Perhaps there could even be a third option of have it initially not expanded, but to automatically expand it when a color is selected with the Dropper. I nearly always want the full Color menu when I sample a color.

At least to me, it isn't clear what you want to do. I'm not sure what you mean by a "mask" in this context. I think you need to explain more clearly or more fully. Perhaps it would help to describe the type of image you'll start with, and what you hope to end up with. Maybe you could list the steps of the ideal process to achieve your desired result.

A more detailed description would help, but perhaps you have in mind pyrochild's Liquify.

I think what you need is BoltBait's Paste Alpha plugin.
 
Copy the black-and-white image to the clipboard, then run Paste Alpha on the color image. The default mode of Paste Alpha is to use the intensity of the clipboard image to determine the alpha of the canvas image (the layer you're running it on). That's what you want.

Something that you might try is using Recolor Using Palette, with the colors you want to use added in consecutive entries in PDN's palette. You could use TR's Custom Palette Matcher instead. It's very similar, but gets the colors from palette files instead of the built-in PDN palette. Both plugins replace each image color with the most similar palette color.

The most obvious choice is dpy's Rotate Text.
 
I believe this will likely give better results than typing in horizontal text, then rotating it. Rotating thin lines tends to result in aliasing (jagginess) or blurring

There are many ways to do that. The simplest I can think of off hand, without any plugins, is:
Convert the image to Black and White using the built in adjustment Add a layer above the image layer Set the new layer's Blending Mode to Multiply Set the Primary Color to the desired color Select the entire new layer with Select All Use Fill Selection to fill the layer with the Primary Color Merge the new layer with the the image layer.  
Before merging the layers, you'll likely want to use the Hue/Saturation adjustment on the color layer (especially to increase the lightness), and the Brightness/Contrast adjustment on the lower layer (to increase the brightness and decrease the contrast).
 
Other Blending Modes when used with the method above, such as Color Dodge, Overlay, and Screen, can also produce nice monochrome effects. Besides Hue/Saturation, the color layer's effect on the image can be modified by changing the layer's Opacity.
 
You can also make the color layer the lower layer, and change the Blending Mode of the image layer.
 
I believe the example image may have a slight blur applied to it, to help soften it and push it into the background.

There are many ways to do that. The simplest I can think of off hand, without any plugins, is:
Convert the image to Black and White using the built in adjustment Add a layer above the image layer Set the new layer's Blending Mode to Multiply Set the Primary Color to the desired color Select the entire new layer with Select All Use Fill Selection to fill the layer with the Primary Color Merge the new layer with the the image layer.  
Before merging the layers, you'll likely want to use the Hue/Saturation adjustment on the color layer (especially to increase the lightness), and the Brightness/Contrast adjustment on the lower layer (to increase the brightness and decrease the contrast).
 
Other Blending Modes when used with the method above, such as Color Dodge, Overlay, and Screen, can also produce nice monochrome effects. Besides Hue/Saturation, the color layer's effect on the image can be modified by changing the layer's Opacity.
 
You can also make the color layer the lower layer, and change the Blending Mode of the image layer.
 
I believe the example image may have a slight blur applied to it, to help soften it and push it into the background.

There are many ways to do that. The simplest I can think of off hand, without any plugins, is:
Convert the image to Black and White using the built in adjustment Add a layer above the image layer Set the new layer's Blending Mode to Multiply Set the Primary Color to the desired color Select the entire new layer with Select All Use Fill Selection to fill the layer with the Primary Color Merge the new layer with the the image layer.  
Before merging the layers, you'll likely want to use the Hue/Saturation adjustment on the color layer (especially to increase the lightness), and the Brightness/Contrast adjustment on the lower layer (to increase the brightness and decrease the contrast).
 
Other Blending Modes when used with the method above, such as Color Dodge, Overlay, and Screen, can also produce nice monochrome effects. Besides Hue/Saturation, the color layer's effect on the image can be modified by changing the layer's Opacity.
 
You can also make the color layer the lower layer, and change the Blending Mode of the image layer.
 
I believe the example image may have a slight blur applied to it, to help soften it and push it into the background.

There are many ways to do that. The simplest I can think of off hand, without any plugins, is:
Convert the image to Black and White using the built in adjustment Add a layer above the image layer Set the new layer's Blending Mode to Multiply Set the Primary Color to the desired color Select the entire new layer with Select All Use Fill Selection to fill the layer with the Primary Color Merge the new layer with the the image layer.  
Before merging the layers, you'll likely want to use the Hue/Saturation adjustment on the color layer (especially to increase the lightness), and the Brightness/Contrast adjustment on the lower layer (to increase the brightness and decrease the contrast).
 
Other Blending Modes when used with the method above, such as Color Dodge, Overlay, and Screen, can also produce nice monochrome effects. Besides Hue/Saturation, the color layer's effect on the image can be modified by changing the layer's Opacity.
 
You can also make the color layer the lower layer, and change the Blending Mode of the image layer.
 
I believe the example image may have a slight blur applied to it, to help soften it and push it into the background.

The pattern, in case it's not implicit from @ardneh's comment, is that each layer's opacity is divided by its distance from the bottom, starting at 1.
 
So for five layers:
Top:   Opacity 51 = 255/5
Next: Opacity 64 = 255/4 (rounded)
Next: Opacity 85 = 255/3
Next: Opacity 128 = 255/2 (rounded)
Base: Opacity 255 = 255/1