Hi there
This question may seem simple but I'm use to doing difficult questions but how would you reprsent this question as a tree diagram:

A box containts two white balls, two red balls, one black ball and one green ball. One ball is randomly withdrawn then replaced and another withdrawn, and its colour noted.

I am not sure, but this may help:

Call 2 white balls W1, W2, and two red balls R1, R2.

Now proceed as usual.