I'd try a higher nitrogen fertilizer like urea, 46-0-0, or ammonium sulfate, 21-0-0. Melons are heavy users of nitrogen.

I won't use the very high P, 10-30-10, very often for anything. You don't want P to get too high and throw things out of balance.

The yellowing could be from other causes like cold soil, below 68F/20C, or too wet.

Since you are in zone 2 your problem with melons may well be cold soil. You can help that some by putting black plastic down around the melons and cover that with clear plastic. Once the melons cover that it won't help much. You may need to raise the temperature in the greenhouse. Your nights even in summer may be way to cold for melons.

How warm is your soil?