Hi there I’m refurbishing and old post for you to have some info in how to mix paints for airbrush use. This comes from personal experience, so please feel free to use under your own risk.

Paint consistency must be mixed to achieve the correct proportion that depends on the surface you are using:
-For high absorbent surfaces as paper the Airbrush acrylics are quite liquid and are absorbed fast. It seems to me commercial Airbrush inks acrylics have the pigments in smaller grain as the normal paints. Are the same pigments as the tube acrylics, but far more processed before mixing into a solution, so they are easier mixed with water. When you thin them in water you can use them as watercolor over paper. The proportion is 1 to 3 (but depends the color) Darker colors may take 1 to 4. And of course you can use the amount you need as measure. Thining with water is ok, because the pigment will be retained in the surface and the water absorbed and evaporated.

-For canvas or non absorbent surfaces the mixing is different, since water is not absorbed, the medium must contain something that bonds it to the surface when is applied. The solution is to use a combination of substances to make the paint thin enough, but still sticky to the surface. I make a combination of Acrylic Retarder, Acrylic medium (Matt , Neutral, or Gloss under preference), water and the normal Acrylic paint.

I normally use for the mixtures Schmincke Products because they are easily found in Germany and are really good quality, but liquitex, createx and others have similar products. Regarding Water, I use water from the bottle, drinking one, but not tab, in Germany contains a lot of calcium and minerals.
Normally I use this products for the mixture:
50553 Acryl Fluid-Medium, seidenmatt
50556 Acryl Retarder
Normal from tube opaque Acrylics (W&N mainly)

Yes the Fluid medium is milky and white, but will not dry that color. Dries pure transparent and gives the end look.

So now the proportion comes according to color and what are you using it for:
For normal opaque Airbrush use, I make a proportion of:
3 ml Acrylic color, 2 ml retarder, 5 ml fluid medium and I start with 8 ml Water but depending on how opaque and color and amount of layers I want to apply I’ll add more water. I start with a lower amount because you can always add more if needed, but not take it out.
For backgrounds, I want thicker paint so I use only 8 ml Water in the mixture.
For second coats, 10-12ml H2O.
For Glazing 15-20ml H2O.

I actually add H2O until I get the consistency I need because it depends on the color. I don’t add it to my airbrush before being sure of the consistency, so I test it with a brush and a piece of paper. Dark colors are quite rich even with a lot of thinning, these are easy to measure and mix and to pass through the Airbrush. Colors as white may be problematic, yet a couple of tries makes the mixture in the correct consistency.

I mix them inside syringes, where I add the substances with enough precision. I use some washers inside so when I Shake them the Paint mixes. I use 2 or 3 of them, these do not rust (very important) and are flat allowing the paint go out fully.
Here the materials:

Here is a mixture example:

I shake them for a while and then I use the needles to pass through the mixture. Because I use them for Airbrush, then if it passes without issue through the needle, it will pass without issue through the nozzle :D..
Here After mixing:

Actually after mixing them like this paints can be stored for months and be reused… You will see when the paint decants after weeks. I always mix new ones to airbrush because else it forms grooms and clogs it, but the rest of the paint I can use it for retouching, or the brush, etc… I have residues from many paintings I use for backgrounds or small stuff… Just keep reusing until the last drop, I’ve thrown so far only tubes of green which is a problematic color..
Here the needles:

Here my Normal Mess, reds and oranges I have them for months now:

Needles need to be clean else they will clog as the acrylic dries. Use caps to hold the paints and leave no air while storing. I have always a water syringe which I use to flush the needles.

Also its really nice when you have mixes because you can always make the same tone by mixing same quantities. For example the cute snake painting I mixed 7ml cerulean blue and 3 ml naples yellow several times and as I need it… I mark the proportions in a tape for later remixing.
Here how to mix:



After shaking Ready to use :

Using this way of mixing paints and colors you can always replicate the same color without failure. All these comes from my experience and by experimenting, use it under your own risk…
I hope it was useful and have fun :D:D