Advice on turtle shell

[donut]

Hi Guys.
My daughter has a few short neck turtles.
They now have a very dark patterned plastron (lower shell) but were light colored.
My turtles have light colored plastrons.
The turtles were all purchased at the same time.
They are about 15 mths old.
Does anyone know why hers have changed color and is this a concern.
Thanx heaps.
Donut

 

[kakariki]

I'm not sure about your turtles but here's a great link. All the info you could want.
http://www.australianfreshwaterturtles.com.au/forum/index.php