There are 2 ways to apply a transparent layer of acrylic color. One way is a "wash" or "stain" which is 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 are usually made on absorbent surfaces. If your surface is matte (not glossy) it is absorbent. I use the word "wash" to signify alot of this diluted mixture sitting on the surface puddling up. While 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 water in any way, 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 need a non-absorbent (or glossy) surface to apply 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 it, it will be 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 using a rag I spread the color thinly and evenly, which works better than using a brush for spreading.
One more idea would be to first apply a thin layer of the Acrylic Glazing Liquid over the surface, then while that is still wet, you can apply colored glazes, which will glide a bit easier.
Additionally, Golden's new Open Acrylics have a very long drying time, and make glazing very easy. You might want to try them instead of the traditional glazes with the more fast drying regular acrylic line of paints and mediums.