help with hair color

I used to get my hair colored in a 2 step process--first all of it dyed one color and then highlights added via a frosting cap application. My problem is now I have several inches of new growth with gray and can no longer afford to have my hair professionally done. What can I use to do just the one process--in other words color the gray and the new growth while blending the old? I am referring to an over the counter hair color like Clairol or L'Oreal.ThanksJanet