[Coloring Selected Color]

I am having trouble figuring out how to color a selected color only. It’s where you have a primary color and a secondary color. The primary color is the color you wish to color over and the secondary color is the color you want to appear over the primary color. I forgot how I did it but I’m supposed to do something where no matter where I press on the screen the secondary color won’t appear anywhere else on the screen expect on the places that’s the color of the primary color.

[Looking for Plugin]

Not what I'm after. I wanted a plugin that lets me easily select around the object in the image. A plugin that auto selects around the object just from me clicking on it once.

[Looking for Plugin]

I'm looking for a plugin that will enable me to easily select around a character or object in a picture and copy just the object but not the background it's on. What I'm wanting is just the object on the picture but not the background. Is there a  plugin like this?

[Stopping Paint.NET Layer Glitch]

I noticed that whenever I try to post my art of a person or object onto a background the lines around the object or person look glitched showing it outlined in white lines. How do I get Paint.Net to blend the two layers without glitching?