No matter what you've read, 20 gallons is FAR too small for 7 tree frogs. Ideally, you should have 10 gallons per frog when dealing with gray tree frogs.

I know they say that background color makes a difference, but of my four frogs all housed in the same tank, one remains a constant bright green, another varies between green and gray/white, the other 2are gray to almost black sometimes .

The humidity stays 70 to 75% temperature between 60 and 80 degrees Fahrenheit. I use a 50 watt daylight bulb for my live plants and it is working wonderfully, it's really not that difficult
You seem to be doing research, you'd be able to find good plants for a vivarium.