The gray gels are "
neutral density" made for lowering
intensity without affecting color/
color temperature. If you're trying for monochrome, then you need to considerably limit the wavelengths of light coming through your
gel (or, multicolor LEDs can be really good at this). That's easy to do with a very saturated primary color, but the physics and physiology of the matter make it tough to do and still wind up with something that reads as white-ish light in the manner of B&W/sepia film. I think the best way is to coordinate with costumes, makeup and scenery to make everything the same hue - specifically a hue that closely matches whatever hue or range of hues you have in performers' skin. Eliminate hue contrast in the
pigment on
stage as completely as possible before you light it, and then you don't have to perform a miracle with the lighting - just pick something that does a reasonable job at killing color without being too saturated. Look at the
SED curves in your
swatchbook for clues and then try a few. R99 and L17 work okay for that sepia look, but then again if you're already making everything on
stage some version of "skin tone" then you might find that alters what color of light is needed to produce the same
effect.
When designing with no hue contrast, value contrast turns into a very big deal. Everyone's got to be in communication designing costumes that don't disappear into the scenery and so on. Lighting can't repair all of those issues either.
