GTPS can start their color change from as young as 2 months to as late as 18 months or so. I have had babies from the same clutch vary quite a bit so there is no hard and fast rule as to when they'll change.

Now, the yellow on the dorsal area is probably going to fill in as it completes the change with mostly green, possibly some blue where the baby patterning was and depending on the genetics possibly some white spotting.

That's one of the beauties of these snakes is that you really never know exactly how they're going to turn out until they're completely changed