I like purple, coral line color, pinks and purples. You might want to look for pieces that have creatures on them, feather dusters, ect. We got a cool crab once, he never came out of his rock but would eat raw shrimp. One day Sinner dipped the rock in fresh water, crab still didn't come out, then he broke the rock in half...I don't remember what it was or happened to it.
Sometimes you can get baby stars on your rocks. we got some and didn't know what they were till later.
I go for interesting pieces rather than 'rock' shape ones...it all depends on what you want. Have fun!