Why light mixes backwards

Paint works by subtraction: pigment absorbs wavelengths, and what bounces back is the color you see. Mix more pigments, absorb more light, drift toward brown-black. Light works by addition: every fixture you bring up ADDS wavelengths to the room. Mix more light, get closer to white. That's why red + green — mud in paint — makes clean yellow on a dance floor.

Here's the twist: a physical gel is actually a subtractive device. It sits in front of a white-ish lamp and absorbs everything except the color it passes. But the moment two gelled beams overlap in the air or on a body, they add. Your filters subtract; your rig adds. Isaac Newton started this whole conversation in 1666 by splitting white sunlight through a prism and showing white light contains every color — then recombining it to prove the point.