If my wordings are confusing, these should be self-explanatory enough:

GG-> b 1/4 * 0 = 0

GB -> b 1/4 * 1/2 = 1/8

BG -> b 1/4 * 1/2 = 1/8

BB -> b 1/4 * 1 = 1/4

ans = (1/4) / (0+ 1/8 + 1/8 + 1/4)

= 1/2