I go to a salon and have them color the whole thing using foils aka the tin foil looking stuff they use for highlighting.

The key is to keep the color off the scalp. So even if they don't get the color tight to the "root", it still will look like scalp since its blonde, the color of our scalps.

I use a higher density, so even if the color does bleed onto the "roots" or base, you can't really tell.

For a lower density, here are other techniques that people use on the boards for coloring. I know if you wanted to color it while its not on your head, you can put conditioner at the root to protect it from getting colored.

Depending on the density of your piece, the foiling method works well. I think if you have a lower density, it might not work out the best since you might have some staining.

What kind of style or color were you looking to achieve?