Typically done in layers with layer masks for CMYK printing but as a single layer example
RGB (additive colours) use the brush in lighten-only mode
CMY (subtractive colours) use the brush in darken-only mode
Edit: Looking for old script that makes good examples - one of Saul Goodes - sg-decompose.scm
It decomposes into layers either RGB or CMY - find it in the layers menu.
Attached, remember to unzip it.
RGB (additive colours) use the brush in lighten-only mode
CMY (subtractive colours) use the brush in darken-only mode
Edit: Looking for old script that makes good examples - one of Saul Goodes - sg-decompose.scm
It decomposes into layers either RGB or CMY - find it in the layers menu.
Attached, remember to unzip it.