Has the frog's behavior changed in any way, i.e. loss in appetite, lethargy, etc..? Have you changed the temperature and/or the humidity levels of the terrarium or have you noticed any changes in these variables? Has anything else in the frog's environment been changed?

It would be helpful if you could post some pictures to further assess the situation. Frogs normally change shades of lighter and darker colors, (due to lighting, temp, and humidity conitions) and some species are known to even change their markings. Based on the information you provided, it is quite hard to determine if this "bad" or not. I hope it is only normal.....

Take care
-Autumn