Colored anglers could be anything. I actually received a wartskin once as a "colored" angler, but don't count on that happening. You could take your chances with a "colored angler" and if it outgrew the tank, upgrade or find it a home.
If I were you, I'd look for one at a LFS, maybe ask them to order you one. Two reasons. 1) Anglers aren't the best shippers, and 2) some species are really cryptic, meaning they never come out, and you hardly ever see them, so what you get is an expensive crapshoot.
One reason wartskins are popular is that they are out and about quite a bit.
An inexpensive angler that is out a lot, attractively colored, and easy to ID is the sargassum anglers (Histrio histrio). They get to be about 7 inches as adults, so it might outgrow a 20 eventually. Sometimes striped (striated or hispid) anglers as mislabeled as sargassum anglers. But these get about the same size and are also pretty active and stay in the open.
See Zubi's frogfish site for more great pics and other information (including size) of frogfish species.
http://www.starfish.ch/frogfish/spec...o-histrio.html
You can usually find the Sargassum frogfish for $25-40, cheaper than most anglers, and you will know what you are getting.
If you keep an angler with damsels or clowns, they will end up being expensive food for the angler. Also, that would way overload a 20 gallon tank. Not sure any fish are safe with the mantis, even if it is a smasher rather than a spearer, especially in a small tank.