I get my hair colored a chocolate brown it's a rich color, I use the boxes when I can't afford the hair stylist, off and on during the yr.
I have white hair roots to hide..so here's what I do I do it as a retouch because you've already had hair color on before. So follow the touch up directions (this will help your hair not to turn out too dark). Even if it's 2 months I still follow the touch up directions.
At the end when you can put the color through out all your hair I use more and put a little water on my hair to help really spread it around. Leave on according to the directions then rinse in the shower (however you like to rinse) and you should be fine.
Don't worry too much I used to work at a salon and yes redkin is great but there's some pretty good stuff out there. You will be fine and even if the color isn't exact since you've waited to grow a bit from last color it will work out fine.
I like going to the hair stylist best because her color no matter what box I use her's doesn't fade so much...I still get the white roots and I've been looking at them for a week now but the rest of my hair hasn't faded.
My stylist doesn't get upset she even say's there some pretty good stuff out there for a box.