No, you do not need cyan, although you can use it if you want. All colors can be altered by adjusting two of three filters only. Basically stated, primary colors of light are unlike the opaque primaries in that there is some overlap. Primary in this case means those from which all others are made, rather than those that cannot be made from other color, like with opaque colors. If you want to control red/cyan, you add or subtract Y/B and M/G in equal amounts. The two can be combined equally to have exactly the same effect as the third filter, but without adding neutral density, which you would be doing if you used all three. (Equal amounts of Y, M, and C equal neutral density, AKA no color, only tone. Just stack Y, C, and M filters together to see this.) You can use any two filter colors that you like, but the common choices are magenta and yellow, which is easier to reason out in your head, as they are both warm filters. It is also convenient because they are the two used for black and white VC printing.
Last edited by 2F/2F; 10-09-2008 at 04:05 PM. Click to view previous post history.
"Truth and love are my law and worship. Form and conscience are my manifestation and guide. Nature and peace are my shelter and companions. Order is my attitude. Beauty and perfection are my attack."
Years ago, I made a color contact printer box with an 11x14 opal glass for the surface one puts the negative on. I painted the inside of the box white, and drilled a hole in one side for a small dichroic color head-held in place with a bungee cord. I arranged white foamcore baffles inside at a 45 degree angle to reflect the light up to the opal glass. I used a common light meter to adjust the baffles to achieve even light on the opal glass. I then constructed a lid on a hinge, with foam rubber glued to the underside, and a latch. To use, the negative is taped down to the glass with black photographers tape, and then the remainder of the glass is taped off with black photographers tape, then an "L" shaped piece of cardboard is used as a corner to reference the paper to, and this is taped down. In the dark, a piece of RA-4 paper is run up against the corner, the lid is closed and latched and the enlarger timer is triggered for the exposure. Works like a champ. At one time I did fairly large quantity (100 prints) each of some product shots with a litho negative of product description type at the bottom of the image. Of course the color negative (8x10) had to be trimmed to allow for the inclusion of the type at the bottom. A composite image, made with one exposure. Worked great. I found that I had to reduce the output of the color head by taping black tape over part of the light output to get exposure times long enough to be reasonable.