As the petals of a red rose turn from the light, what color will the red petal become?
What is the color of a shadow cast upon a bright red ball on a field of grass?
I find these exercises extremely useful.
In watercolor there are several variables that can alter the result of the mix. Always keep in mind the value of the pigment at full strength. More water will create the effect of light reflecting back into the shadow causing it to be a lighter value, but still neutralized. Whether the pigment is opaque or translucent alters the results. The amount of pigment used to gray the red will determine whether or not the object retains red as its local color. Too much of the pigment used to neutralize the red will turn it either too purple, too brown or too gray. It will no longer look like a red object in or out of the light.
I like to see the subtle differences between the mixes. When working with a limited palette, it’s good to know what the possibilities are. I should have included Ultramarine Blue and Cobalt Blue just for reference, even though they are too far from being complements to be considered as pigments for graying vermillion. I try to keep in mind that graying is neutralizing not simply altering the intensity of a color.
I prefer graying pigments using near-complements rather than perfect complements, since perfect complements often take the life out of a color, neutralizing too well. By using near-complements I can suggest the lighting situation and nearby objects reflecting color back onto the object while at the same time creating harmony with the other palette colors.