Plaster is way too breakable to use anyway. But I buy cement colorant. You mix some with the water and then mix it in the cement. Comes in several colors. I get mine (in a small box) at Home Depot. But actually I prefer to paint my stones anyway. You can use regular housepaint or the craft paint, just seal afterwards. I use concrete sealer as it's really worked great for me.
Here's ones I painted, plus the two reddish terra cotta-color stones I used the concrete colorant instead.

{{gwi:129941}}
Karen

Purplemoon, those are gorgeous stones. Each animal looks so life-like! Thanks for the tip on the plaster.

You should stick with the pre-mix stuff and DO NOT worry about any of the latex admixtures. TRUST ME. My DH has been in the decorative concrete business for more than 10 years.

Do not use old cement. It WILL cause trouble down the line. Use up your bags of mix ASAP, don't use some then put it away for a year or you'll either have a hard rock or you'll end up with crummy concrete pieces.

I like to paint my concrete, or you can use wood stain to give it a nice look. Do this once your piece is cured out...at least a few days old to ensure the paint won't peel off.

If you're in California, you shouldn't have to worry about stones cracking, so no need for the latex stuff. Go as cheap as you can for better results. 8 years ago I made some stepping stones for my mom using the quick mix stuff. She's moved four times and taken those stones with her and they're still just fine.