The obvious choice would be the "Object ID" pass but it's broken at the moment ( I think K.A.Schubert is trying to take care of this ).
If you want to color correct objects but also depending on the shader, better to directly use the "shader ID" pass .
For unlimited number of mattes, use "render Layer ID" pass.
Though the primary purpose of the "render layer ID" pass is to easily control the contribution of the render layer dedicated passes ( Shadow and layer reflexion pass) .
But you can use "Render Layer ID" pass as a custom matte pass, assigning different render layer ID at whatever object you want.
And if you want only RED GREEN and BLUE mask, only use Render LAYER ID 73 , 18 et 36.
73 =
Red
18 =
Green
36 =
Blue
It's a bit tricky to get a "render layer ID" pass, details here :
viewtopic.php?f=28&t=53027
Also for this workflow better to render this pass alone with the Info Kernel, render much quicker.
UPDATE: better to use the
object layer colorpass now instead
).