I have heard both stress and colder weather can bring on darker colour. Your tank is only 11 gallons, and usually the rule is a min 10 gallons per frog.... but whites being bigger tend to need more. Tree Frogs also like height and usually 24 high is recommended. Maybe price a 18x18x24... its 33gallons and you might be able to get away with 2 whites frogs or a really comfy 1 XD. what is the heat and humidity levels at? also do you have lots of hideing and climbing spots/ branches/ fake or real plants. this can all reduce stress.
Taken from the Whites care sheet on here:
Temperature Day: 29 °C (85 °F); Night: 20 - 24 °C (68 - 75 °F)