I can't seem to get a dark brown using food coloring.?

I have red yellow blue and green food coloring dyes and I want to create a dark brown food coloring for vanilla icing, I tried using yellow blue and red but it only gives me a grey and if I remember correctly when I mixed all of them it did not give me brown. I don't want to use chocolate frosting but that's the... show more I have red yellow blue and green food coloring dyes and I want to create a dark brown food coloring for vanilla icing, I tried using yellow blue and red but it only gives me a grey and if I remember correctly when I mixed all of them it did not give me brown. I don't want to use chocolate frosting but that's the color I want please help.
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