I do a lot of work with 70s screencaps as well, and soft light layers (especially blue-coloured ones) are v good for counter-acting the eternal beigeness of cardboard telly - but the MM tutorial is actually a pretty good look at how you can build up colour using lots of different kinds of layers, which I should probably remember to try myself. (I tend to do the duplicate in soft light x2/3 then duplicate screen layers x2-3 and adjust and then work from there.) It's hard to get them lighter without things getting grainy or washed out, but it can be done!