If anyone wants to replicate this problem… (one other poster ‘marcus_civis’ also could reproduce the problem)
"1: Open up ZBrush and select the default cylinder and make polymesh 3d. Then I draw it onto the canvas and go into edit mode.
2: With standard brush selected I select MRGB, turn off Zadd, and select basic material. With my main color RGB:255 (meaning R:255 G:255 B:255) I go into color and click fill object. I then switch back to RGB with rgb intensity set at 100.
3: I then subdivide the cylinder 5 times.
4: I then click switch colors and adjust my main color to RGB:2 and paint a bit onto the cylinder.
5: I then adjust my main color to RGB:72 and then paint over the white and the black in the same stroke.
I then can see that the black area that I painted over with the grey brush is slightly darker than the white area and no matter how much I try, I can’t paint over the black area to match the brush’s color.
I used the C hotkey to sample the colors. The grey that is painted over the formerly black is RGB:71 and the grey painted over the white is RGB:72, like the brush. I checked to make sure there’s no mask or layer inadvertently active."
