The problem occurs in color mixing situations:
- painting with soft or partially transparent brush
- using blur
- having a gradient
- having partially transparent layers.
It can be seen even at anti-aliased color borders because they are partially transparent.
Darker mixing result of bright colors like max. bright cyan and red seems to be caused by the nonlinear gamma property of the common RGB color space.
In Photohop's color settings (=Edit > Color settings) you can force RGB color mixing to happen with linear (=1.0) gamma value but that's not an option in Krita.
An easy workaround is to select linear gamma RGB color space when starting a new image or to convert an already started image with Image > Convert Image color space.
Conversion doesn't unfortunately fix those dark mixes which are in a single layer, but it prevents making them more. Mixes caused by layer transparency and effect masks are fixed by converting to linear gamma.
You get a summary of the selected color space properties if you click the color space browser button:
If the color space is the inbuilt sRGB you have nonlinear gamma (=the white curve). With it you get nearly black looking edges if you paint say bright cyan on bright red with a soft brush:
If you select a linear gamma version of RGB the problem vanishes:
The used color space is this:
The depth is 16 bit integer per channel. Actually one can select depth = 8 bit integer /channel but Krita warns it can cause harms. With linear gamma too high proportion of the available 256 levels is used for dark colors and that can cause banding because well visible bright colors are too sparse.
Conversion from 16 bit depth to the usual 8 bit sRGB with nonlinear gamma is possible when you are ready. It doesn't bring the dark edges back if your image is flattened to a single layer before conversion. But if your image has separate layers where colors are partially transparent the dark mixing results return as soon as you change back to the usual nonlinear gamma, so be careful!