Except you may not get true ND; there may be a filter cast introduced.
Originally Posted by Bob-D659
The idea works well if you are using a negative translator, or happen to have a Kodak set-up photometer, which will automatically filter out any bias in the filter settings. By bias I mean that if you had set the filter dials to 45Y 50M, you might find, if you compared the readings on your translator with the filters in and out of the beam, that you actually had, say, 41Y 53M. The problem is that filters age, they fade and chip, and get dirty and dusty, and there can also be mechanical problems with the cams and tracks used to position them in the enlarger head.
In one operation I worked at in the past I made internegatives, as well as Ektachrome dupes and Type C prints on the same enlarger. All the darkrooms in this operation had really nice Minolta-Zyco translators, and there wasn't a single enlarger head, out of about 15 darkrooms, which tracked with 100% linearity. One enlarger I had in my darkroom was fitted with four filters; cyan, magenta, yellow and neutral. When making small density adjustments using that particular enlarger, dialing the density wheel would cause very small (1-2 CC) colour shifts, so it wasn't 100% neutral.
BTW, cyan and yellow filtration are used for Ektachrome printing and when making duplicate transparancies on Ektachrome and Fujichrome duplicating stocks. I have also used the cyan filters when printing a negative exposed under florescent lighting; under those circumstances, you end up removing all of your magenta filtration out and adding a not-inconsiderable ammount of cyan filtration.
But Bob-D659 is correct; for the most part, don't touch that dial!!