It sort of works the same way as q3map_alphaMod.
q3map_rgbMod volume flags it as a editor volume brush that you use to place over vertexes to control the RGB vertex colour values, much the same way that alphaMod alters vertex alpha.
q3map_rgbMod set ( R G B ) sets the vertex colours of the enclosed vertexes of the volume brush.
colorMod and colorGen are the same thing.
On that note, the development version of Q3Map2 has the following new/fixed features, available now:
q3map_colorGen -or- q3map_rgbGen
q3map_rgbMod -or- q3map_colorMod
q3map_alphaGen
These new keywords are synonyms for q3map_alphaMod, except where color requires 3 values:
q3map_rgbMod scale ( R G B )
q3map_rgbMod set ( R G B )
which is the same as
q3map_rgbGen const ( R G B )