If you're going to use water-based acrylics, I recommend using a wet palette. If you've never used one, a wet palette consists of a water-tight and hopefully air-tight container, a sponge, and a piece of porous material as the actual palette. There are papers made for this purpose. You wet the sponge to a little more than dampness, lay the paper over it and let osmosis do the work on thinning your paint. A wet palette allows for uniform consistency or thinness, and it lets you save a batch of colors from one session to the next.
When I decided I wanted to use one, I made my own out of a take-out container, a kitchen sponge that fit into it, and I used brown packaging paper for the palette. I saw the advantages right away, though the negative to my rig was the paper deteriorated rather quickly, getting fibers in my paint. And eventually the lid cracked around its edge and was no longer air-tight. But since I was going to move to a commercially available wet palette, that wasn't a problem. I could have used palette paper with it, too, just cut to fit.
Masterson is one brand, available from art supply houses and at craft stores. I bought RedGrass Games' original palette in the small size, about half the size of a sheet of paper. I liked the smaller footprint compared to the larger ones. I use it with all my water-based acrylics, from Andrea and Vallejo Model Color, to Lifecolor and Reaper, to craft store brands like Folk Art, Americana, and Apple Barrel.
Now, to your original question about Vallejo colors. First, let me say that Vallejo Model Color is formulated for hand-painting and is very popular. I think for that reason, not too many painters have noted colors being off. As Hazebrouck mentioned, they can be blended, and I think that's what the connoisseur painters do. They will create highlight and shadow shades through their process.
I don't blend mine too often, since my toy soldier style doesn't really call for it. I will mix colors to get a particular color of shade of a color. But otherwise, they're good for hand-brushing. The only complaint I have is that some of the colors can dry to a glossy finish, even though they are formulated as matte colors. Vallejo's Prussian Blue will dry glossy, for example. Again, for my toy soldier style, that's not a problem, because I finish my toy soldiers with a coat of Future for a gloss finish. For someone painting in connoisseur style, he can always add a matte medium, or use a matte varnish to seal when finished.
As noted, there are a lot of water-based acrylic brands, including brands sold to the wargaming community. Reaper is one, and I think Warhammer has its own brand. No reason not to use those, either, except that "Burnt Siena" might be called "Dwarf Barf" or something like that in a wargame paint catalog.
I don't have any AK or MiG paints, not because they're not good, but because I already have a stash of paints and I don't feel the need to add yet another brand. Unless you remove the cap and the paint magically finishes the figure, I'm not interested in adding it.
One more thing-remember that "acrylic" does not mean "water-based". It means the paint's binder is an acrylic compound. If the paint is water-based, ie, water-soluable, that means its binder is water-soluble. There are acrylic paints whose binders are alcohol-based or lacquer-based. You can't use those on a wet palette, because they need other solvents to thin them. There were even acrylic enamels, particular the old Pactra brand. Those were water-soluable, or could also be thinned with isopropyl. I still have some of those, 45 years on.
I hope that helps!
Prost!
Brad
