"What are you (and others) using for the blue of the trousers?"
I will bet a penny to a pound I am the only one doing this but hey it works for me. The photo has bleached the colour, especially the highlights, quite a bit too so I think it looks a lot better in the flesh so to speak. So this will not be of any use to you but here goes
Base coat of a colour called "true navy" which is an Inscribe craft paint.
Wash of Pynes Grey water colour (which I also use to line in as its not as harsh as black)
Back to the base coat for first highlight and then build up by adding an off white in small amounts, I think I used Foundry Linen C.
Svennn