I think "Testors" was an awfully big tent. When we were very young, Testors made little square bottles. Many years later, they fired up a sub-brand called Modelmaster that had more colors specifically mixed for particular subjects (all except WWI).
That Modelmaster line has been discontinued, but the little square bottles remain. Many are the flat very basic colors (flat=best for hand brushing by non-experts), some gloss and some fluorescents. Today's little square bottles cost much, much more than the ten cents we remember, but that's the way it goes.
Today I was in a shop that had nearly all of the current line of little square bottles and I found that I needed to put my glasses on to read the small print about color and flat/gloss split, marked on each bottle.
Personally, I don't like to mix specific colors because I rarely finish a model in the same year as I start it, and I don't want to try to mix the same shade for my touch-ups. So I find the best color I can, of paint that brushes well for me. There is a game store near me, and I am using the Army Painter Warpaints line, which is intended for painting wargame miniatures, and comes in horrid "dropper" bottles. I have been making samples over white or grey Tamiya spray primer to see how the paint behaves and how close it is to my desired target. (I do my sampling on big sheets of sheet plastic - For Sale or For Rent signs from my local hardware store. (I'm using US terminology - sorry if my words don't match yours)
And as far as "old paint" goes, don't toss them just because they are old. I try to take care of my paint bottles and tins and have been using some recently which I bought in the 1970s. Give them a try before tossing. And keep the threads on the bottle and cap clean.
Good luck
Rick in Seattle
