There is no single answer to that question. You are correct in assuming that more magenta = more contrast, and that more yellow = less contrast. However, each paper has its own response to filter changes. You can't expect the same result from say 30M with Ilford paper as you would with Kentmere or any other. The fact sheet that comes with the paper should give you some starting points for the more popular color heads. Some use a combination of magenta and yellow in an attempt to make the exposures between settings consistant.