I am afraid you may have to live with the red. Nickels are pretty fragile. I have a medicine bottle full of red buffalos. Pretty much just the way it is. Of course someone may have a method out there somewhere.
Are some finds, you might try letting the nickel sit in mineral oil for a day or two to soften any residual dirt.
I tend to do this with most of my non-silver, it helps bring out the details and preserve the coins.