Pastels tend to go through several color changes as they age...
(In Most) From hatchling the yellows tend to get brighter for a while, then into adulthood the yellows will usually start to fade out to some degree and continue with age. Also if you notice there are the brown out scales on top and coming down the sides that look like they cover the clean yellow scales. These Brown out scales will continue to brown out with age.
As for lineage (NERD Lemon, Graziani, ect.) it really does not matter. Selective breeding of any line can produce top notch pastels. The parents are more of an indication of what you will possibly have as your baby gets older. A Bright Clean Pastel Bred to a Bright Clean Normal offers a better chance of producing Bright Clean offspring. If the parents are clean bright animals you have a better chance of a clean bright offspring. On the other hand there are browned out low grade pastels produced from every line.
Pictures could help with members giving you an idea of what to expect. There are many of us who can spot certain characteristics on the younger pastels and tell what will happen to some extent with age, but NO one can tell you exactly what your animal will look like as an adult
As for all Ball pythons Darkening with age there are a few Mutations that get brighter or cleaner with age like the Fires... If you want to produce Bright Pastels breed a Pastel to Fire and try to produce a FireFly