Chiming in! I actually opened this thread before anyone had replied, then had to go help DS with homework. Came back, refreshed, and voila!
1. First, what color are your natural roots closest to in the RC color swatches?
2. Do you want to hide your gray completely, or do you want it to blend like natural highlights? (that's what I do.)
3. Yes, the color will take darker, so you will want to choose a shade about 2-3 shades lighter than you actually want.
Out of those choices, I would go with Coffee Brown, being the lightest. I used to use it, but switched to Golden Chestnut because it was getting too dark and I wanted a highlighted effect.