Acrylic Glazing

GLAZE and Refractive index
Acrylic Color = Pigment + Binder
Refractive index of the Binder is 1.4 for every Acrylic color but that of Pigment varies from pigment to pigment, and closer the Refractive Index of Pigment to that of Binder(i,e. 1.7) , more is the transparency/transculency. Example: Refractive Index of TW is 2.7 so it is opaque and that of Utramarine blue is 1.7.
What light manages to pass through or between the particles is ultimately absorbed or reflected by the lighter, underlying ground, gaining a second opportunity to interact with the pigment particles before making its way towards the viewer. This complex interplay of light within the paint film creates the particular sense of glow and luminosity that is one of the prized attributes of glazes.

Why use GEL instead of WATER?
Binder 's refractive index plays a critical role in limiting the scattering of light, and having the particles well bound in a paint film becomes crucial for creating a luminous translucent glaze. For this reason a glaze needs to be made with plenty of medium to insure the pigment is fully suspended. A 10:1 ratio of binder to color is a good place to start your mixtures. A very common mistake is to simply thin the paint with water, which only leaves the pigments more exposed to the air, where the scattering of light is greatest. Rather than a smooth translucent film, the paint will also tend to create a rougher, matte surface that further dissipates the light and leaves the color feeling washed out.

Loss of Light
Lastly, it is important to remember that glazes will always lower the value of the underlying color. Less light, not more, is ultimately reflected back from a glazed surface, so it is important to keep the underpainting or ground at a higher value then you anticipate for the final result.

Mix or GLAZE?
Glazes allow for a unique development of color that cannot be achieved by any other means. A bright yellow, glazed with a transparent red, will result in an orange completely different than if they were mixed together on the palette. There is a sense of luminosity and greater saturation of color caused by the light traveling through one or more translucent films before reaching the viewer.

Paraphrased from http://www.goldenpaints.com/justpaint/jp12article2.php