Hello, there is a way to paint with rainbow on Krita?
Yes there is.
First, select the fill_circle:
Then, press F5 to open the brush editor, and toggle Hue.
This should allow you to change the color depending on the pressure.
The brightness of the rainbow is relative to the color of the currently selected color, so make sure to select bright saturated colors for a bright rainbow!
Uncheck Pressure and check Distance to make the rainbow paint itself over distance. The slider below can be to change the value with keyboard input.
When you are satisfied, give the brush a new name and save it.