It sounds like moisture us getting underneath the gray nail. This happens when your product seal is not sealing properly. Check to see of there is a crack somewhere on the nail that is allowing moisture to get in. The yellowing could be caused by using a contaminated brush or brush cleaner, or from a product that is not “non-yellowing.” — Tracey Stadamire
Yellowing can occur from using old acrylic product or from monomer that doesn’t have a UV protectant. Make sure you store monomer in a dark, cool place, always use a proper dispenser made for that product, and never mix old and new monomer together. — Michele Martinez
Discolored nails could be the result of several things, including exposure to contaminated liquid, powder, primer, or brushes. Another contributing factor is nail enamel. Make sure to oil buff the nails and use a good base coat prior to polish application. Also, acrylic that has not been sealed properly by oil buffing (or buffing with a three-way buffer) will be porous and can become stained by the pigments in enamel. — Marti Preuss
Most acrylic products will “age” and discolor. Yellowing is not unusual. A definite spot of yellow-orange on the nail could mean that the primer is touching the existing acrylic during a fill. Gray nails are usually caused by contamination or the use of a “dirty” compound. — Sue Roberts
You need to determine whether it is the natural nail or the acrylic that is discolored. Discolored natural nails could either indicate poor circulation (gray-purple) or surface stains from nicotine, polish, etc. (yellow-orange). Discolored acrylic product indicates either the use of inferior product (old liquid or no UV protectors) or poor technique (mixing liquid with primer or not using fresh liquid with each service). — Jaime Schrabeck