It's a vector variable so you write it:
lamMAT_matrix.color = << r, g, b >>;

where r = red, g = green and blue ranging from 0 to 1.
Black would be << 0, 0, 0 >>, white would be << 1, 1, 1 >> and mid green would be << 0, 0.5, 0 >>. Get it?