Mark,
More information would be helpful. Are you looking at weathering reds and blue's, or Highlighting/Shading the same colours, or even changing the tone?
I guess that this is a question with regard to the use of complemetary colours to neutralise a primary colour. This could be used to reduce the vibrancy of the primary colour & even effect weathering or shading. In the case of blue you could use orange & in the case of red you could use green. The type of paint is immaterial & I would suggest experimenting a little until you achieve the effect you're after.
years ago I attend a painting from the my club where greg talked about graying down colors including british scarlet I believe that is red but he used a lighter mixed a dark green and then mixed that with a scarlet and got what I called a washed out red but all I am asking is it possible to do the same thing with Vallejo or Reaper Paints ? or any other paints ?