Marble Mollies Turning Black?

[Shuvee]

I have owned two marble mollies now and both of them went from black and white to nearly solid black. Is this a common phenomenon? What is the cause?

I also own a dalmatian molly but she has retained the coloration present when I first bought her.

I apologize if this has been discussed before but I could not find a clear-cut answer browsing the internet. Thanks in advance.