There are two ways to apply a transparent layer of acrylic color: decreasing the power of the color by using water or by using medium.
The first way, using water, is accomplished with a “wash” or “stain” made by using a mixture of water to colored paint in a ratio of about 8:2 (this isn’t an exact science, but the idea is to add enough water that the acrylic binder is completely diluted, usually at least half water to half color). This makes a very diluted color which sinks down into the surface of the substrate. Washes and stains, therefore, are best applied on absorbent surfaces. If your surface is matte (not glossy) it is usually absorbent. I use the word “wash” to signify a good quantity of this diluted water-paint mixture which will then sit on top of the surface, often puddling up. I use the word “stain” when the diluted mixture is applied, then quickly rubbed into the surface with a dry rag, so only a hint of the color remains – like a “stain”.
The second way to apply a transparent layer of color is by glazing. A glaze generally does not involve any water, but instead uses a mixture of medium to paint color in a ratio of 8:2. (again, not rocket science, so feel free to play around with the ratio – but again at least half the mixture should be medium). By using medium in the glaze (instead of water as in the washes), glazes will sit on top of the painting surface and therefore are best applied on a non-absorbent (or glossy) surface, which will allow the glaze to be applied evenly and easily.
At any point in a painting’s process, when you feel the need to apply a transparent layer, take a moment to look at the surface absorbency. If it is matte then try a wash, if it is glossy then use the glaze. If it is matte and you would rather use a glaze, then first apply a coat of a gloss medium. Let it dry, then apply the glaze. The reverse is true too. If your surface is glossy and you want to apply a wash, then use some product that gives a transparent grit. My favorite for this is to use Golden’s Acrylic Ground for Pastel, diluted at least 1:1 with water. If you don’t dilute this product, it will be gray and opaque and may slightly veil or obscure the paint layers underneath.
Other tips: I like to apply glazes with a brush in very small areas at a time. Then before it dries, I take a rag and spread the glaze color thinly and evenly, which works better than using a brush for spreading.
For ease in applying a glaze, try first applying a thin layer of the Acrylic Glazing Liquid (or some other slow drying acrylic medium) over the surface where you want to apply a glaze. Then while that is still wet, apply colored glazes into the wet medium. This gives a longer time to spread it out, eliminating streaks.
Additionally, Golden’s new Open Acrylics have a very long drying time, and make glazing even easier.
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