Saving a png combines similar colours

[VT45]

So I'm working on a topographic map of a size that I've worked on before, but something strange is happening I've never encountered before. When I go to save it as a png, I lose colors and therefore contours. So for example, I have this section of map:

But when I go to save it, the save configuration window displays this:

When I go to save it, the darker green colour is lost, as is reflected in the save configuration window. I have never had this happen before, and I have no idea what's going on. How can I fix this?

[null54]

2 hours ago, VT45 said:

How can I fix this?

 

Change the Bit Depth drop down from Auto-detect to 24-bit.

 

The Paint.NET auto-detect code is picking a 2-bit (4 color) palette, and the Microsoft code that generates the palette is making some bad color choices.

[IHaveNoName]

I was wondering whether VT45 was using the default save as PNG or the more sophisticated "Optimized PNG" plugin. 

[MJW]

6 hours ago, null54 said:

The Paint.NET auto-detect code is picking a 2-bit (4 color) palette, and the Microsoft code that generates the palette is making some bad color choices.

 

Png is supposed to be a lossless format. I seems to me that when auto-detect is specified, 2-bit color should only be selected when there a four or fewer colors in the image. Anything else seems like a bug in auto-detect

[Rick Brewster]

Palette generation, which is used when auto-detect determines that 8-bit (or less) is possible, has been funky in various ways for awhile. I've been working on the fix-of-all-fixes for the 4.2.16 update.