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http://www.ilford.com/html/us_english/pdf/Cont.pdf
If you go to page 3 of this PDF (and since it's a PDF and I couldn't just copy and paste) you'll see a section titled "Dual Colour Filter Settings." I'm not touting this as true (and I note now that it says "...but should need LESS adjustment to exposure times when changing contrast." not "no") just passing the information along I had read.
Let's see what I've got in the magic trash can for Mateo!
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Not for nothing - but - "......Adding magenta to a yellow, only cancels part of one out. say you have a 65 yellow and want to add 20 magenta. What you are effectively getting is a 45 yellow. It will not effect the density at all. just dial in the 45 yellow......."
is not completely true. Most multi-garde papers are now made up of two emulsion layers, and as stated, each activated by one of the above filters. Mixing these will produce a contrast (if not density) different from using only one or the other. Another example is the design behind most VC cold light heads where two bulbs change intensity of mixed color as you shift contrast.
With that said- David - Why not teach yourself the difference with the; paper, enlarger, etc. you are using by running comparitive test strips. Seeing the difference will help you understand the materials response to these changes.
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This discussion had me curious so I went and fiddled with the contrast controls of my Aristo VC Head. Cool little devil I can dial in contrast settings in 1/10th grade steps.
At the lowest contrast setting the light is blue, but moving through the range you can see the colour of the light change from blue, blue-ish green to green. So there is a mixing of light occuring from the two tubes. Never really paid attention to this as it just magically works.
- Mike
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Mike,
I think it is the other way around isn't it? Full on Blue should be the highest contrast and green full on the lowest contrast.
lee\c
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Lee:
Of course you are correct. Just went into my darkroom and turned the beast on and checked again.
That's my second senior (dumb) thing I have done today. It's either lack of sleep or dementure setting in. Of course could be both 
- Mike
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