This makes the random vertex color brightness work on a relative basis.
Because humans see relative differences in brightness more then absolute
brightness, simple adding randomness to `Value` will cause dark colors to be
effected more heavily then light colors.
This also fixes a problem desaturated colors with becoming red after
saturation randomization.
Pull Request: https://projects.blender.org/blender/blender/pulls/107315
cdccc51b2e