Multi matte, colored alpha, color mask there are alot of different names for this technique. The vRay multi matte render element lets us easy and with relative little effort setup a multi matte. To use the multi matte do the following.

Using shader to define ID

• On the shading group enable atributes>vray>Material ID and set it to an int.
• In the render setting add a multi matte render element.
• In the multi matte render element be sure to enable the use material id checkbox to make use of the mat id we setup in the first step.

To use more than r, g, b add another multi matte render element.

Using object id to define id

this can be done on single object and multi.

Single object

• on the object shape node add attributes>vray>object id set id to an int.
• add a multi matte render element.
• set the object id to corresponding int.

Multi objects

• select the object you wish to separate.
• create>vray>apply single object proprties node to selection
• set the object id of the single object properties node
• add render element, use corresponding object id.