Steven:
My method is a little different but it allows you to control the final 'color' of the concrete as you stated.
Whether it is wood or plastic I start off by sanding with 80-100 grit paper to roughen the surface (if you use textured paint as the last coat you can skip this step).
Next comes flat black paint - about 80% coverage
Next comes red oxide paint - about 80% coverage
Then satin/flat dark gray - almost 100% coverage
The coat will give you the concrete shade you want. I use - satin or flat
Light gray or white, or Desert Bisque textured. You can also experiment with the other top coats cited above.
I find that the three base coats gives me an excellent starting base for the final top coat. You end up with reds, and dark grays. This gives the top coat some subtle color shades when you are done.
