Sad News, Happy News, and Interesting Observation.

The sad news is that one of my African Dwarf Frogs died about a month ago. Wanting to get a pair and keep them properly is what started my interest in keeping tanks with fish and frogs. Worse, the remaining frog started calling after lights out almost constantly since losing its tankmate.
The happy news is that when I went to my local lfs yesterday for food, they still had a handful of ADFs from their last shipment left. They don't seem to get ADFs often (maybe every 6 months?) and sell them pretty fast when they do get them, so I wasn't expecting to be able to buy one anytime soon. I was a little surprised when I got it home and discovered it's almost half the size of the original frog. I hadn't realized mine grew so much. Oh, well, regular tweezer feeding (to ensure neither frog gets to hog all the food) will fix that soon enough. Also if one frog is trained to come to tweezers for food, apparently it doesn't take long for the newbie to get over it's fright and head straight for the tweezers and the hand holding them.

The interesting observation has to do with the frogs' coloring. My tank has pale sand as substrate (not the best choice and I'll know better in future). The store has black gravel in the tank where the ADFs are kept. Because ADFs were what got me wanting a tank, I all but haunted the fish store waiting for them to get a shipment, and bought the original pair as soon as they were out of quarantine. Those frogs were pale when they arrived at the store, and stayed a medium/pale brown in my tank. I'm pretty sure I remember seeing the shipment my replacement frog arrived in and the frogs were pale colored on arrival. The frog I just brought home has darkened in color to the point where it is all but black.
I observed something similar in the Daisy's ricefish I bought from the same store. The fish I bought that spent little time in the store tanks have pale adult coloring. The two females I bought later to even out the sex ratio and spent their growing period in a tank with black gravel are much darker. In fact, a year later, their color hasn't faded any. Unfortunately I don't have the space to experiment, but it would be really cool to know just how much influence on color intensity tank substrate has.

Absolutely. Fish and invertebrates are well known for assimilative colouration to suit the environment. There are other strategies however, but in the cases you mention it seems very plausible that they match background. Other strategies are defensive (intense colouration) to warn like many brightly coloured frogs in Amazon. Schooling is another defensive strategy, whereby a few of the population are sacrificed for the betterment of the group. Truly intresting, fish behaviour is linked to colouration, linked to survival mechanisms.

My ADF -second generation born in the same tank-have color variation from light greish-brounish to the much darker colors.
Feeding with the tweezers sounds interesting. What do you feed them?

Frozen bloodworms, and a frozen food that is supposed to contain a mix of glassworms, brine shrimp, algae and bloodworms. I can't find ADF specific food, so I try to offer as much variety as I can. They seem to eat everything I give them.
The tweezer feeding started when I first got them and they were really skinny. I wanted to be sure they each got their fair share. It came with the side effect that they started recognizing that the hand meant food, and not being scared of it (unless I move stuff to clean). At one point, one of them even swam into my hand and sat there as it tried to be first to the tweezers.

Whenever we bring in trucks of tropical plants from Florida you are bound to find little lizards and tree frogs. They will both change their colour depending on what they are sitting on.

Steve