You just have to use some logic:

She has 8 red ones, 4 green ones, and 4 blue ones.

In the worst case the first 8 jellys would all be red, and the next 4 would all be either all green or all blue.

It is possible to have taken out 12 jellys and still not have all colors.

But once she takes out lucky 13 there is no possible way for her to NOT have all three colors.