Why do my dalmation Mollys turn black once I get them home in the tank?

Question
Hi,  I have a 10 gallon tropical water tank and Im curious as to why when I get a dalmation molly from the store they are pretty with lots of white,  after being in my tank for a week or two it turns almost completely black.  Are they only pretty and colorful to a certain age or is this something I am doing to make this happen.  I have a water tester kit and it says hard water , low- moderate alkalinity, and somewhere between 7.2-7.8 ph. all other levels are in the safe zones.  Any advice you have would be greatly appreciated.  Thanks

Answer
Hi,
Sorry for the long time in replying. I did not realize I had a question.
I wouldn't worry about it. When I bought my Dalmatian Mollies they were spotted black and white, and now they are both completely black. It often happens as they mature, and it can't be stopped. It's perfectly normal though.
How many do you have? Be careful not to overstock, and don't have more than 5. Your tank is big enough only to support 10 inches of fish, and they grow to 2 inches each.
If you have any more questions, please ask.
Best of wishes,
Kathryn