This happens to me every time I go to a pet store!
When at Petsmart, I was choosing between two bettas; a beautiful lavender Plakat, and a red and teal feisty crowntail. I ended up choosing the crowntail, because the Plakat really didn't look to healthy, and since this was my first Betta after getting back into the hobby, I wanted to start off with a healthy one. While I'm happy with my boy, I wish I could have boughten the Plakat as well!
Another time at Petco I found a lightly marbled blue/purple veiltail with light yellow fins. He was gorgeous, he reminded me of a watercolor painting. I was so close to purchasing him! However at the time I didn't have enough money to go out and buy a whole new tank, heater, etc. :/