Xenopus can change color to some degree as Geoff stated with his article.
Have you thought about feeding your clawed frogs nightcrawlers? The bait shop variety from walmart work fine, I feed my 4 xenopus these and they go crazy. Since you've had your frogs for so long I have no doubt you know how to care for them, though yeah the current water volume is too little for this species.
I think once you've moved them to a large tank things will improve, these guys are messy and like having a lot of space to swim around. The temperature swings are a bit much here, try to keep the tank at 70F or close to that if possible, 65F is a bit cold for these guys and 80F is completely too warm. A larger volume of water will give you more stable water temps, as the large the water volume the less prone to 'swing' it becomes.
I think these frogs will be fine, just move them in to a bigger tank. Make sure they have a cycled tank (take your old filter and move it to the new tank), give them adequate filtration and keep up on your partial water changes with clean, tap water treated with a dechlorinator. Try live foods, clawed frogs simply cannot resist earthworms.