Hi wilomn,
Sorry your having trouble.

Since yesterday, when they looked fine, one has turned very dark, almost black
Color changes are anything from normal to illness . There are many reasons for this: mood, humidity, temperature, stress, illness?

drizzled white glue - it may need to shed ?
Check the humidity level! Be sure to have a shallow water dish ( water not above it's chin ) with de-chlorinated water for it to soak in-- available at all times. Cleaned daily .

They're rarely active during the day
Red eye tree frog are NOT active during the day! They sleep all day. They are nocturnal and wake after the lights are out.

Excessive activity ( restlessness ) during darkness could be a cause for concern.

Care sheet: ( for humidity , temp, lighting, food and supplements- etc)
http://www.frogforum.net/tree-frogs/...caresheet.html

Please answer these questions so we can continue to help you:

1-size of enclosure
2-# on inhabitants - specifically ( if there is another frog ---size differences ?)
3-humidity
4-temp
5-water - type - for both misting and soaking dish
6-materials used for substrate
7-enclosure set up i.e. plants( live or artificial) wood, bark etc -how were things prepared prior to being put into the viv
8-main food source
9-vitamins and calcium ? ( how often )
10-lighting
11-what is, specifically, being used to maintain the temperature of the enclosure
12-when is the last time he ate
13-have you found poop lately
14-a pic would be great ( including the frog ) any little cell phone pic is fine
15- how old is the frog
16-how long have you owned him
17- is the frog wild caught or captive bred
18- frog food- how often and if its diverse what other feeders are used as treats
19- about how often the frog is handled
20-is the enclosure is kept in a high or low traffic area
21- describe enclosure maintenance ( water changes, cleaning etc)
22- has or was the frog properly quarantined

Hope this helps
Lynn