To begin with the choice of colors would top the list. So what family of colors is on your mane wish list? Are you looking to alter the color or color the gray?
The second stop on the internet is an Expert's page....most likely to be on lifestyle web sites which tell the ground rules of which skin types goes with which hair color etc. Don't bother to read too many of these as each one of them is likely to have their own version of hair color palette! Once you have short_listed a range of hair color options, try looking for pictures of people with similar coloring and how they would look in it. Many salons have sophisticated software which helps conjure up an image of you with the hair color and it might be worth the money you spend on it.