I'll throw my vote in for bettas, too. My first, Mr. Betta, looked pretty drab in the store when I bought him-- he looked a dull shade of blue all over, and I really only bought him because I felt sorry for him and wanted to give him a proper home I knew he wouldn't have much of a chance of getting elsewhere. Within a week of being in a proper tank, his true colours started to really show. The bland, lifeless blue blossomed into a lovely deep royal blue with a metallic sheen, and he started to grow a wonderful vibrant red on his fins near his body, too. He's a stunning fish now, and he certainly knows it and is very proud of it, haha!