I know at Michael's they sell a four pack of Wilton gel colors that are Toy Story colors.
I agree with Ez and recommend experimenting a bit so you get a feel for the color, and it'll give you hints on mixing colors in the future.
As for me, I mostly use buttercream at this point (still new to fondant), but when I needed that color I'd start with a lime green that was about half yellow and half green, if not a hint on the green side. And then add white as needed (which didn't turn out to be that much).
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