This image demonstrates how to get different colors using two layers--one red on clear and one blue on clear. The third, on the far right, is both of the two pieces of glass together. With the addition of yellow paint, or silver stain, one can get red, blue, yellow, white, dark purple (or near black), orange, green and lighter hues. The principle is pretty simple but can be confusing to think about. Wherever you want blue, green, yellow, or white: sandblast the red all the way off. Wherever you want red, orange, yellow or white, sandblast the blue off.
A color image such as this above can be made with the following steps. I find its best to work one plate all the way through so as not to get an incapacitating brain freeze trying to orchestrate the color relationships. So start with red and follow these steps. Remember, these aren't the laws of God inscribed on tablets and delivered to us mortals by Moses. These are just the way I do it. Sometimes. (Click the images to enlarge)
First three steps of the red plate. Stage sandblasting and flex shaft engraving.
Second three steps to completion.
First three steps of blue plate.
Let me know if you have questions!