Greetings,
In this image

I started with a red line, green line, and blue line.
And applied Colors==>Map==>ColorExchange to each line.
It exchanges colors, except for the outer edge.
You can see the outer edge remnant.
How can I get a more complete color exchange, including the edges?
Thanks...Vernon

The edge are "anti-aliased pixels". If you paint directly on a background, the edge pixels are made of pixels that are a mix of the subject color and background color (this avoids the jagged look), so Color Exchange doesn't see them as colors meant to be changed.

You should never paint on another color. Create a transparent layer and paint on that. Then, instead of getting an intermediate color, the edge pixels get an intermediate transparency, but are the same color as the rest. So you can change their color at will (and you can also change the background color).

ofnuts,
I don't understand what you are saying.

Let's say i have an image called TurquoiseMan.png (your avatar).
(Hmm... for some reason, I can't place image 'in line'.)
And lets say we want to change color hex"51FBF9" to lets say color hex"FF0000".
Pls, how would I do that?
Thanks...vm

If you change only that color, only part of the image will change, because there are many pixels that aren't that color. So, if you want to change all pixels to red, juts paint them over with red (you can bucket-fill) after "locking" the alpha channel (the "lock" checkbox at the top of the layers list). But the result isn't too realistic because there is no longer any variation in the output:


So you can instead use Colors/Map/Rotate colors, and define an "input" angle that covers all the variations of turquoise, and map it to an equivalent "output" angle in the reds: