I have just run across a great mixing color to lighten Polly Scale NATO colors. 'Aged white', by Polly Scale, seems to lighten the other colors without the dreaded 'killing' of them. I did not have to add any other colors to it (yellow, for example). Works for me. I also use Polly Scale's Steampower black for most undercoating and darkening mixes. I just really like the way it works.
Happy modelling! capnjock