If a certain color of dye merely needs to be added to arrive at the intended final hue, that is fairly easy, and really not much different than spotting a black and white print. But if you need to do selective bleaching in order to restore a white base prior to retouching, well, that can be pretty tricky. The option is painting over the emulsion using something like Marshall's photo colorants (not to be confused with their dyes). But either acrylic or oil paint is not transparent like a dye, and it's hard to disguise the subterfuge. What Bob just referred to, Photocomping using punch and register technique, requires a fair amount of experience and pricey special equipment. Modest changes can be made by taping a piece of frosted mylar in register with the original and adding tint to that, rather than to the negative itself. Entire books were once written on the subject. Merely reading through them can be intimidating. But along the way, one learns quite a few advanced printing skills which might not meet your immediate need this time around, but could potentially come in handy later. Printing is just like that : you can make it either as simple or as complicated as you wish. Unlike most peoople, if it's going to get complicated, I'd rather do it in the darkroom than on a computer. But I've already got the correct gear.