Are your foils blonde?

I have coloured over blonde foils before but I used a chocolate brown, so something warm. If you get a cool brown, it may go green. Chocolate browns will look slightly reddish. Are you going to do the colour all over? It will go brighter on the blonde parts, possibly a bit red/auburn. I've done this and it looks nice, but depends if you like red in your hair.
You could split the foils up, by weaving your comb through them and keeping, say, half separate and wrapping it in foil. If you dampen the part first that you are wrapping in foil it will be easier to hang onto and wrap up. Then you colour around it and you will get the finer foils that you like. When you shower and rinse of, try to rinse the bulk of the colour off first, before you take off the foil, just in case the colour stains your blonde parts.
I use a Garnier colour from the supermarket. It's a permanent, I think. It's number 3.6, Crimson Promise, and they say it's a Deep reddish brown. This works well on my hair but it is quite dark. Obviously the depth fades, so you could try this and lighten it with a couple of washes.

Good luck with the fix up!