I'd check the frogs belly for red-brown spots. That can mean fungus, and fungus sometimes triggers color change. Ask Amphibian Demon more about it, I only just learned of it.

My juvenile bullfrogs, Gnag [Guh-nagg] and Bumpy, have been brown for quite a while, ever since the first week they were in the new 30G. Now, in their previous tank, they were nice and green, but only because there was a ton of moss and plants [probably because]. In the 30G, they've only got green plants and lilypads along with fake water reeds. They usually hide under a small log, and that, along with the gravel, is probably the reason they're brown.

My suggestions for you are:
Get a UV (B/A) lamp from the pet store.

Maybe add more green foliage if you want the frogs to be green.

Well water is not a good idea, in my opinion. Rain water and dechlorinated tap water [If your area uses Chloramine, don't try to de-chlorinate the water. I'm not even sure how to de-chloramine water.] or [What I do] Pure Spring Water bought from a store [Make sure it is not distilled water.]

It could be contamination from the surroundings, so you might want to clean the tank...

I'm FAR from a frog expert, I'm just offering my opinion. Ask Heather or GriftheGreat if you want expert help on this.