Created attachment 88085 [details]
Screenshot of the brush test area demonstrating the bugs

Attempting to reproduce the brush effects presented in the documentation (https://userbase.kde.org/Krita/Tutorial_4B#Color_dynamics, https://userbase.kde.org/File:Krita-tutorial4-IV.2-1.png)
gives unusable results.

Example 1a:
1. Select the default brush and pure red color (#FF0000).
2. In Brush->Color enable Hue variation, pick distance, set it to 300px, no repeat.
3. Effect: a streak of dark grey for a while, then pure red.

Example 1b:
1. Change the curve so that the endpoint is below half of the height.
2. Effect: dark grey streak for a short while, then pure red.

Example 1c:
1. Change the curve so that the endpoint is at 100% and the start point is above the half height mark.

Example 2a-c:
1. Same settings as with examples 1a-c, this time with saturation.
2. Effect: the transition appears to be smooth at first, but in the first two cases the color suddenly jumps to black. The red is never pure and appears to transition to some off-turquoise color.

See screenshots for the demonstration.