It's completely natural for your hair to be more gray than previously, and it also makes total sense for you to want to look your best.
You may want to call around to some of your local salons to see if any of the colorists have experience working on cancer survivors hair. Other than that I'm sure a professional colorist probably will have experience in this area, given how high the breast cancer rates are (unfortunately).
I think once you have "corrective coloring" done by a professional (they use toner to set the color (which doesn't come in the kits you buy in the phjarmacies) you'll look more like you wish and feel much better.
I color my hair (don't want to be gray) and I didn't have chemo-just vain!...;)