What paper are you using?
If it is variable contrast paper, the magenta filtration will increase the contrast, while the yellow filtration will decrease the contrast.
The third filter is a cyan filter - it will have no effect on the contrast, and is rarely used at all.
There is a white light position on the controls that moves the filters out of the light path. If it is engaged, changing the filter settings will have no effect.
If everything is working correctly, turning the magenta filter control higher will make the light look more magenta.
The same is true for the yellow filter control (light will look more yellow) and the cyan filter control (light will look more cyan).
If you have more than one filter control set to a number, the resulting light will be a combination of them all.
I have a pdf of the manual for a Beseler CIII, including the colour head. If you would like it emailed to you, send me an APUG pm with your email address.