Second, your tree frogs are beautiful.
Unfortunately, I have to critique a little bit... You should not be mixing species. Even if they were all the same species, that's way too many animals for a 20 gallon tank. Gray tree frogs have their own toxins that will effect the other animals. They might not seem stressed yet, but after several months, it'll start to effect them. Now, on to the meal worms, that is not a healthy diet for the tree frogs. It'll fatten them up, but with very little nutrition. Also, you risk impaction by feeding mealworms. It's okay on occasion, but your better, healthier bet would be crickets.
I have 4 gray tree frogs, they are all in the same enclosure. It's 35 gallons and decorated with a lot of greens and browns. I have 1 tree frog that stays bright green, one that varies from white to green, another that goes anywhere between nearly black and white with green spots, and another that stays totally gray and black. I don't know exactly what affects their color, but it doesnt seem to matter in my vivarium, they all take on their own colors.
It seems like you really care about the health off your pets, so I would definitely get each species in their own enclosure and get some crickets. You will have some very happy pets then!