I'm an artist with a job to do a painting on an exterior cement wall and I want to make sure I use the right paint. It was previously painted and is in decent shape. I could use cement paint, but it doesn't come in all the colors I need. Can I use cement paint for the first coat, and then exterior latex for the painting itself? Is there a clear protective coat available?

Yes, yes and yes.

Give yourself a good couple coats of a top quality masonry paint as a base.
Lightly sand the base coat then use a top quality exterior latex (or better yet, oil) for your art.

There are a variety of clearcoats that I've seen at Sherwin Williams, PPG, etc but have never used any of them. I don't see why polyurethane wouldn't work too... but be sure to ask a professional first (or hang here a few to see if someone else pipes in with experience on clearcoating over paint).