Good write up - the hardest thing with contrast paints seems to be finding the right colours, epically for military subjects. The painted colour can vary a fair bit from the colour in the bottle, let along the colour you see online. And as you found the undercoat colour has a significant impact too.
I’ve found them quite useful on 10mm figures too - the more details in the surface of the figure, the more chance the paint has of giving a ‘contrast’ But even just for good coverage they work well and quickly. I’m more of a fan than I expected to be, and I often used thinned paints over white undercoat before contrast paints. The advantage of contrast paints is that they cover well.