What type of paint works well on plaster?
Since you don't want to loose any surface detail in your casting and it's a porous material you are wanting to colour, have you considered using a dye or stain instead of a paint?

Using dyes, you chemically change the colour of your material. Adding no thickness to the item being coloured.

I don't have much experience with dyes other than colouring Easter eggs. Using melted wax to mask areas allows for ornate designs to be created. 

Here's a demonstration of the process.