Many painters know, if one adds too much solvent, usually water in acrylics, the paint will break, where the medium/solvent and pigment separate ...
(forgive the quality, I could not be bothered to set up lights etc)
So, above, we see a selection of GW standard range paints, which have been diluted, 1:1 with water, and left to stand. They are quite usable (for the job they were chosen to do), after shaking, but the breaking is obvious. This can also be seen, quite naturally, for some Vallejo MC paints, on the rack, in any store.
Now, the following observation came about, when I was looking for a dark blue, base coat for my SM/Chaos army. After trying many stock colours, I finished up choosing GW Foundation Mordian Blue. So, I diluted it (1:1 water, 15 drops of W&N Acrylic Flow Improver, in 20 ml, as usual) and after a few days, this happened (daylight bulb) ...
(all my paints and mixes have a good quality, stainless steel, nut in them for shaking)
The white precipitate is obvious, which sticks to the bottom of the container. I presume that this is the 'special' additive, which gives the Foundation range its good coverage. I also suggest that the pigments are liquid, but that is just a gut feeling. This also occurred with the AFI absent.
The point is, that the resultant upper layer of paint is perfect for what I need. 'All' pigment, it flows beautifully and is diluted to the consistency I like for 2 or 3 coats. Mechrite Red also did the same, but the precipitate is not so obvious.
Incidentally, I tried a Vallejo MC near-equivalent, Dark Prusia Blue ...
(like the back of my hand)
The first is my 'magic', GW Mordian Blue 'mix', the last is Vallejo MC Dark Prusia Blue, diluted, 1:1, with retarder/AFI/water mix. What is interesting, is the middle colour, which is the one on the right, heavily-diluted with the same additive mix - look at the colour, more Royal Blue !!! - just goes to show :)