As plaster of paris is extremely porous, I would suggest you use a sealer first. As for the medium itself, I would use Gouche [pronounced wash]. I believe it's still sold in ready to use form, some in a solid cake, like water colors, and some in tubes.
For the aged look, I suggest you experiment using "payne's grey" to mute the colors. You could also experiment using an overglaze in a neutral.
You might go to the library and get a copy of "The Artists Handbook". Lots of good ideas and info on handling and mixing of colors.
Edited by red clay - 13-Nov-2014 at 11:22