Thanks. The way I do it is after I carve the foam out I first go over it with a heat gun. The heat causes the foam to wrinkle just a tad so it gives a nice realistic rock texture, but the added benefit of that is it also hardens up the foam a bit so it is more durable. After that then I do a coat of drylok (I use white drylok as the base coat). After the drylok then I go over the entire thing with black paint. The black works its way down into the crevices and helps to add some depth the the background.
Once it all dries black then I start doing the color coats. Usually with grays, browns and a little tan or lighter brown in spots. After that then I do a bit of dry-brushing on the high spots to add some "wear" to the higher spots of rock that would normally show some wear in nature. Then after that I coat the whole thing with Pond Shield.
That's pretty much my process for doing background. That's not to say that's the only way though, just what I started out doing and works for me. I do know some people used cement tint and add that into the drylok directly.
You could tint the drylok and not seal it if you wanted. The sealer does for sure help with durability, especially if you have plecos or fish that graze on algae that grows on the background, but you could still get quite a few years out of it without sealer. I just like to use sealer to make it as durable as possible so I can not have to worry about it many years down the road is all.