When looking for a top notch pastel or axanthic you should look at the end of the scales, if you see a black tip on the scales (I call that tipping) then you can anticipate the snake will only get darker with age, if you don't see black tipping then you should hold that snake back as a future breeder. It's real simple hold back your best looking babies then later breed from them. After you've done that for 3 or more generations you will begin to see consistency in the way your babies look.
IF I were you I would add fire, gold blush, vanilla, yellow belly or orange dream to any pastel project, those traits (and there are more) will help to enhance the quality of the yellow scales.
Good luck
Brian