Acrylic can discolor when exposed to sunlight or UV tanning rays. Some discolor faster than others depending on the "recipe" the manufacturer uses. Most discolor slowly enough that the older acrylic is being removed as you shorten the length of the nail during a fill. Many manufacturers have a finishing top coat that acts as a UV blocking agent. Do not confuse this with a non-yellowing top coat.
Discoloration can also come from chemicals being sandwiched in between new and previously applied acrylic. The most common culprits are primer and base coat when doing a fill. Ensure that primer is used very sparingly and avoid getting in on the previously applied acrylic. You can also better remove chemical residue from the surface of the nail by using polish remover that contains a little acetone, since this will cut the polish better, and by roughing up the pre-existing acrylic during prep. – MaeLing Parrish, Nail Sensation in Columbus, Ohio