Alumilite opaque and clear resins will yellow over time if it is exposed to UV light. Clear acrylics or lacquers work well for protecting the resins from changing color. There are two ways of applying paint to achieve the best adhesion. First, paint the mold and allow it to completely dry before casting your resin. As the resin flows into the painted mold, it will chemically bond to the paint as the resin is curing and after the resin cures you will de-mold a perfectly painted part. Painting the mold prior to casting allows you to reproduce perfect detail by painting becoming the outside layer of the actual part rather than cover the exact cast replica with paint that could then cover up fine detail. The other option for painting your part is to paint the part as soon as you remove the cast part from the mold and after you remove any flash while the resin is still curing. Although the resin is demoldable, the casting resin is still curing and if painted during the first 5 or 10 minutes of casting the part, the resin will still crosslink with the paint to give you not only a mechanical bond but also a chemical bond between the paint and the cast piece. Paints have built-in UV stabilizers which then block out the UV light from ever reaching the casting resin which could affect the color.