I've always just accepted the fact that when printing colour you have to dial in filtration as just that - an unquestioned fact and never thought any more about it. It was only last night when I was lying in bed that I started to wonder why we actually need to use filters. I mean we don't use filters to expose the negative in the first place (well ok I know we do to correct colour balance, etc under certain circumstances but you know what I mean.) so why do we need them when we print it. Surely if the negative is correctly exposed and the paper is correctly exposed then shouldn't that be it? I can understand the need to perhaps tweak filtration say between differing brands as Kodak film and papers will have different characteristics to Fuji but why can't you just stick a Kodak negative in the enlarger, some Kodak paper on the baseboard, adjust exposure for the required density and that's it?