Here's the Frontispiece from the book Quartz Family Minerals, by Dake, Fleener, and Wilson (1938).

The specimen shows the color effects acheived at Idar-Oberstein. From the top down: dyed fuscia; green, yellow, blue, carnelian, sard (brown) (all beizen), and untreated natural agate.

Note that the process does not affect either the opaque white layers, nor the massive crystaline areas (as in the center). Consequently, a typical result is that the banded structure of any particular stone is enhanced and "picked-out"—though, of course, it would be possible to select material that would accept the processes in a plain uniform manner too.

This book, and other more recent tomes, discuss the coloring processes in some detail (especially recent titles). I recommend reading these before you "try it at home."

Good luck. Jamey