There is nothing wrong with getting a few burgundy or brown mushrooms, except that you may be wasting your money. Your octo may move them or hide them or run over them. I've found that the larger ones like a lot of light, the smaller ones like a shadier place away from direct currents. One thing about them - they move about on their own.
If you think your tank looks too dull, try shopping for some smaller pretty shells. Take a look at the pics of Carol's tanks (corw314) - she uses a lot of shells.
Don't even consider any type of fish. Octos eat them and they can attack an octopuses' eyes and stress the octo.
Nancy