Ilya asked me about Vallejo Triads which combine a base, mid tone, and highlight paint. Foundry made this triad painting style famous amongst wargamers with sets including a shade, main colour, and highlight. Reaper now do the same and probably others.
Unfortunately there is no official source of Vallejo triads and Google didn’t reveal a comprehensive Vallejo triad system. So I pulled a set together from what I could find. These triads use only the Vallejo Model Color range of paints – I think that makes life simpler, certainly for me since I don’t use other paints.
The triads are listed in Vallejo sequence order for the mid tone. Choose your desired mid tone then look up suitable base and high light colours.