I have one layer with two colors, yellow and purple for example. There are some places where they are mixed (anti-aliased edges).

How can I change one color (yellow) to some third color in selected area, so anti-aliased edges remains same?
So, if I have 30% yellow and 70% purple in some selected pixel - it will become 30% green and 70% purple?

Illustration: http://dobrokot.ru/pics/i2016-02-17__05-59-26_68kb.png

Have tried "continuous selection tools", got problem with pixelated antialising (not sure, which "threshold" to choose)

I don't know how to replace colors to exact percentages, but i advice you to look at color balance and color adjustment tools

use the color to alpha on the selection, then select the rectangle tool, use fill foreground, set the blending mode to behind and paint with the rectangle.

TheraHedwig wrote:use the color to alpha on the selection, then select the rectangle tool, use fill foreground, set the blending mode to behind and paint with the rectangle.

Thanks, "color to alpha" is the key feature which I missed!
It worked! I create copy of layer behind main layer, delete color I want to change with "color to alpha", and draw in the layer behind. Then merge.

It has same issue as "continuous selection tool" with unknown threshold, but interactive preview helps to choose the threshold much faster.

The "color to alpha" hides in the Filter/Color submenu.