15 works, but it forces red and green to be the same:

Blue=6, 6+6+6=18

Red=3, 3+3+3=9

Green=3, 3x3-3=6

Blue x Red - Green = 6x3-3=15

Alternative answer would be 20. This allows for negative integers, which lets each colour be unique:

Blue=6, 6+6+6=18

Red=3, 3+3+3=9

Green=(-2), (-2)x(-2)-(-2)=6

Blue x Red - Green = 6x3-(-2)=20