Fading Ambystoma opacum - is this normal?

[igor]

Hi everyone, I have seven adult salamanders (three spotted salamanders, two Jeffersons, and two marbled). We found a young female marbled, Inga, back in March. This is from the day we found her; you can tell her pattern is still "growing in":



As time went on, she developed a lovely silver marbling:





Showing off with her friends Chip (young adult male spotted salamander) and Igor (adult female Jefferson salamander)

In the last month, however, we've noticed that her pattern has faded a great deal. Her behavior is fine: she's alert and eats, though her appetite isn't quite at the level it was in March/April/May, when it was colder in the house. Our other marbled salamander, an adult female, has not faded, and we aren't sure why Inga is. The pictures tell the story:







Does anyone know what is going on here and if this is normal?

Thanks in advance for your input!