The colors will change as they grow, some colors will get more intense and some colors will fade. Though usually there is some indication of what color they're going to be as adults when they're small. It really is best to get one where you can at least see the parents. Otherwise you're going to be in the dark about what you're going to end up with.
My first dragon came from a petstore and was mostly brown but her stripe down her back was slightly red. She ended up being bright orange and red, which is great but she just as easily could have been brown.
You just need to decide what is important to you. If you are wanting color, or a certain color then you need to go through someone who can show you the parents. Otherwise go with the dragon who has the best personality or which ones patter you like best. I promise you if you pick the dragon that stands out to you the most you'll think they're awesome no matter what color they end up being. I mean look at this site, every single one of us thinks we have the most beautiful dragon ever. :lol: