In this video, photographer Ab Sesay explains his technique for getting maximum saturation from using gels. If you want to skip the needlessly lengthy video (not all of the meter readings taken were necessary to explain the concept or arrive at the preferred settings, but I’m sure Sekonic appreciated how many times the photographer used one of their meters), I’ll cut to the chase for you – set your gelled lights to underexpose the areas they hit, and keep other light sources (such as an ungelled main light) from spilling onto that area. Voilà, instant saturation. The more underexposed the gelled light is in your image (as long as it is still perceptible), the more saturated it is.
And don’t misunderstand me; a light meter is a very handy device for studio and on-location photography and videography work. However, in this particular instance, I question the value of metering your ungelled lights just to see how much light each gel is blocking. You can likely skip that step, attach the gels and meter them to achieve the desired underexposure (as Sesay mentions, about 1.5 stops under the main light), knowing that different gels will require different flash power levels to achieve the precise exposure you’re going for.
B&H carries Sekonic light meters.