Can I treat with product (such as hairspray, gel, mousse, powder, styling paste, wax, or oil) in my hair?
You may treat while there is product in your hair, as long as the product does not block the light from reaching your scalp, either by contaminating the lens inside the cap or by obstructing the light if it is very thickly applied.
Additionally, the REVIAN cap does not emit UV light, which can cause color fading. Rather, it uses precision LEDs emitting light at 620nm and 660nm wavelengths, which are non-harmful and non-polluting, so if you dye your hair you can treat the same day you color (or any day).