I know this is a really basic and almost stupid question. Can anyone help explain multigrade, specifically Ilford filters to me?

I'm printing on a Beseler 23CIII-XL enlarger with a variable contrast head. This is a link to the head. I'm printing with Ilford MG IV RC Pearl paper at the moment, although I do have some MG Fibre as well. My head has settings for Agfa, Ilford and Kodak papers if IIRC. The Ilford setting goes from green (0) to a dark blue/violet (5). The only instructions I found on Ilfords site say to just set it at 2 and go from there. One of the data sheets also explained that the emulsion on MG paper is sensitive to certain spectrums of light, hence why green vs blue light will help vary the contrast. They offer no other explanation on exactly how or in what situations you would need to use the different filter colors.

If anyone has a link to a basic tutorial I'd really appreciate it. A few hours of searching via google turned up nothing. If anyone can help with a link, or simple explanation of the filters in regard to the specific Ilford paper, I'd really appreciate that too.