Merging colors... How do it with Gimp?

[rich2005]

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.