i have one silver molly and i got it about a month ago... it was quite small compared to the other molies that were in the tank. now it's grown about the same size as the adult molly that i have swimming around. i also noticed that my silver one's dorsal fin is darkening a bit. i have read somewhere that it was normal for their colors to change up as they grow older so you shouldn't worry too much about it. to be safe, just keep monitoring your molly and keep an eye out of any signs of infection.
i stumbled upon this helpful chart that helped me monitor my lil fish's state.
here it is:
<a href=http://www.klsnet.com/files/fishchart.htm>CLICK ME</a>
well, goot luck with you and your fish! this site has helped me out a lot when i was getting started so im glad to be able to share what i know now. i hope that this info helps you out somehow.