This note is about acrylic paints, mostly in miniature painting context.

Acrylic paints consist of:

1. Pigments: the color of the paint, thin liquids with tiny solid particles
2. Binder: "traps" the solid pigment as it dries, usually polymers (resin)
3. Solvent: makes the paint flow and dry, mostly water
4. Additives: other things to change properties of the paint like flow aid or gel agent

"Medium" means "the thing that transports the pigment" thus you can think a medium as binder + solvent + potential additives or "paint without the pigment."

Industrial paint pigments are frequently very thin liquids, not powders. Paint consistency comes mainly from the medium, but pigments can affect it.

Note that each paint can have multiple different pigments in it.

Default to applying thin layers of acrylic paints. If you apply paint in thin layers, you have more control how it'll look and can easily hit the sweet spot of not having too thick layers that will hide all the awesome details of your models. It just takes more time.

Avoid diluting primers, shades, washes, dry paints or technical paints with water. They include specific kind of medium so water changes how they behave.

Thinner medium is paint without pigment. Usually used to make the color less intense, more translucent but maintain its other properties. You have separate mediums to enforce certain properties e.g. wash medium to keep thinned washes flowing and gel medium to keep the thinned paint more thick.

The most useful thinner is... drumroll... tap water. There is rarely a reason to use anything else for normal acrylic paints. Even distilled water is an overkill.

Paint pigments have different transparencies.

Red and yellow pigments are usually less opaque. They will not cover well over black and will look patchy over white.

It might require a layer of something like pink or pale yellow to get a good coverage.

Thinning can radically change the color of a paint. Adding water will always thin a paint and make it less opaque, but it can also change the color as it appear as e.g. thinning brown can cause it to appear more of a yellow brown or even just yellow.

This largely depends on the pigments that are being used in the paint.

Retarder (an additive) prevents the solvent (i.e. water) from evaporating. Effectively makes paint dry slower for e.g. making it easier to wet blend colors.

Flow improver (an additive) reduces the surface tension of a paint. Makes is flow easier off the brush and reduces forming of droplets. Also called a flow aid.

Surfactant (an additive) reduces all tension of a paint. Both tension between paint-air and liquid-solid. This makes it easier to "push" the paint around.

If using Citadel paints, take the paint from the plastic lip under the paint pot's cap. Never from the pot itself as it's too easy to dip your ferrule (metal rim of the brush) to the paint, potentially ruining the brush. Dropper bottles are much nicer e.g. the ones Army Painter and Vallejo use.

Technical Additives

• Liquitex Flow-aid: a strong, artist flow-aid, used to make paint flow more. Mix in 1+20 aid-to-water ratio and add a drop of that diluted version to paints on palette in no more than 1+4 ratio. Can also be used to revive really dry old paints when undiluted. Makes blending easier and hides brush strokes on thick paints.
• VMC Retarder Medium: add a drop of retarder to slow down drying time of your paint, allowing more advanced techniques like wet blending or marbling.
• VMC Crackle Medium: used between two layers of paint to create a crackled surface (first layer will be the background color, and the second will form cracks.) You might want to gently rub some second layer of paint off, and you will have to use a varnish to seal the effect. The thicker layer of crackle medium you apply, the wider cracks you get.
• VMC Thinner Medium: water-based paint thinner, making paint less pigmented but keep the thicker consistency.
• VMC Metal Medium: 1+1 mix of medium-to-paint creates a colored metal of your choosing; or for thinning metallic paints.

Pre-mixed Paint Mediums

General purpose paint mediums that don't add special effects.

• VMC Thinner Medium: water-based paint thinner, making paint less pigmented but keep the thicker consistency.
• VMC Glaze Medium / GW Lahmian Medium: 4+1, 3+1 or 1+1 ratio of medium-to-paint to create glazes. Apply over a surface to tint the color or build gradients with multiple layers.
• AP Quickshade Wash Mixing Medium: use to thin washes and shades, usually 1+1 ratio. Allows the thinned paint to keep wash properties.
• GW Contrast Medium: use to reduce pigmentation on contrast paints while keeping the wash-like consistency.

GW Contrast Medium and S75 Dispel Magic are the best pre-mixed mediums.

Sources

• Composition of Paints