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I normally use a lot of reds and greens. Grey is normally second to either colour. I don't normally use red and green prominently together, that would be too festive, but either one or the other.
I don't think I have a particular color that I return to in all different types of paintings, but as I am still just learning, maybe it just hasn't shown itself yet!
I've found that ever since I bought raw umber oil paint that I have been relying ion it quite a bit as a dark and a cool in my head painting class. If anyone doesn't know what color that is, it is dark brown with a more greenish rather than reddish tint. Other than that I use a lot of cadmium red light it seems, but REALLY, white is the color that I go through the fastest, though that is easy to see why that is.
WearyKnight, red and green actually look quite good together as compliments, and don't look too Christmasy if you don't use them in their pure form.
Blue we be my color of choice. I find that it expresses freedom, and reminds me that I can take flight.