The first step is to figure out what YOU like in a pastel. Lots of blushing? Bright yellow color? Crazy busy pattern?
Once you know that, you can know what to look for in a baby. The pattern won't change at all, so get one you like. Blushing may become more obvious as the snake gets bigger, but it won't appear out of nowhere.
The hardest part to predict is how much the pastel will brown out, but generally cleaner brighter yellows stay that way, and oranges and anything that looks muddy will brown out.