How are color filters applied in different stage effects?

Answer 1

They're just placed in front of the stage lighting (floods, spots and foots), between the light source and the subject. The lamps have attached frames for this purpose. The colored "gels" don't change the direction or focus of the lights, just the color. A true additive mixture of colors is obtained by using additional lights, each with its own color, rather than stacking gels on a single light. Or one can but almost any shade and saturation gel one is looking for. Stacking also has the disadvantage of overheating the gels. Other gels can provide diffusion, lensing and masking. Also, numerous lamp options exist, and dimmers can change both brightness and apparent color. See the refs. (Since this is in the physics section, I assume you're not looking for aesthetic advice, but the first ref. contains basics of both aspects.)