If G=yellow and g=green, and at a separate gene P=purple and p=white, what proportion of your offspring will be green and purple if you cross a ggPp individual with a GgPp individual?

One of each four.

Explanation:

When you cross ggPp x GgPp you get the following offspring:

GgPP GgPp ggPp and ggpp and they are all expected in the same proportions. The breeder wants green and purple offspring, or ggP - and you can see that that corresponds to ggPp which is expected in a proportion of one of every four offspring.