Not sure if this is right, but I tried writing all the possible ways and got this:
gg, gg, gb, gb, gb, gr, gr, bb, br, br, rr, bg, bg, bg, rb, rb
Each g represents a green marble, r for a red marble, and b for a blue marble
there are 2 green-reds (gr) and 18 possible marble color choices, which gives us 2/18 and when simplified is 1/9