Any color fades up over time, bright colors become lighter so it makes since that your underhood would be lighter than fresh paint.

Paint colors are mixed and bad mixes happen all the time, if the whole job is done withi it, it may not be obvious at first until it is beside a correctly color. Good and I mean really good mixers can match faded/mismixed paint colors, very few exist any more since the age of computer mixers.

Find a good mixer, have him do the best he can, shut it and live with what you get. Chances are it wont be the only thing that isn't perfect/correct.