Yellowing applies only to acrylics!
1) Your powder yellows...
All powders contain peroxide. Without this ingredient your acrylic would not set up. Peroxide is a catalyst.
(A catalyst is something that causes something else to happen. You know, like when your client smudges her French manicure for the third time and you get angry and slam your hands down on the desk and knock over your files and you bend over to pick them up and as you do you slip and fall and break a nail! The catalyst in this story was your clients smearing her polish. It caused a chain of events that led up to your broken nail.) Peroxide is like that client, it starts the chain reaction that makes the powder and liquid set up. It is a necessary component of the powder. Unfortunately peroxide yellows and discolors with age. So, all powders regardless of which brand have the potential to yellow. The key is to use as little peroxide as possible in the formula. As a rule, fast set powders that rely on more peroxide tend to yellow more than others.
2) Your liquid doesn't yellow, it just lets the overlay turn yellow...
Any good liquid will contain not one but at least two different types of ingredients as prevention against yellowing. (Its the same principle as sunscreen for your skin...)
A) U.V. Blockers. These prevent or block ultra violet rays from entering the nail and doing damage.
Remember, U.V. light particles can cause a physical effect on your skin by stimulating it to tan. They can also cause a physical effect within the overlay that will lead to yellowing.
B) U.V. Absorbers. These absorb ultra violet rays and prevent them from doing any damage to the nail. If ultra violet blockers fail to perform these absorbers are extra insurance.
3) Are you letting your primer dry?
Primer is an acid. That means that it is a corrosive. If you apply your wet acrylic on top of a wet acid the acid will seep into the acrylic. When the acrylic sets up it will have a foreign, corrosive ingredient mixed into it that causes premature product break down. The break down will present itself as yellowing, brittleness and eventually lifting.
4) Are you contaminated?
Remember to work clean. If you have a dappen dish with liquid in it and that liquid has powder residue from a previous client, chances are that that liquid is already discolored. What are nails going to look like if you start out with discolored liquid? Get the point? Always start fresh and clean. Also be aware of residue from other chemicals such as nail polish remover. (Some people use this to clean brushes. Do not do this. It can cause yellowing.)
(Note: Do not confuse yellowing with staining. To remove stains clue to polish or other causes simply buff the overlay until stains are gone. A highly cross-linked monomer will provide great stain resistance.)