Questions like this seem to come up fairly often. I strongly suggest anyone interested in black and white photography and printing (in colour work this would go without saying) commit a basic additive/subtractive colour wheel to memory (and/or keep a small copy in the camera bag and/or keep one in the darkroom near the enlarger. It is an invaluable visual aid to anyone not familiar with colour mixing/absorbtion, complements etc. It will help with filters in the field, and will help darkroom workers understand what the various filters (additive or subtractive) are actually doing.