I find it difficult to get a light enough spray on cookies using airbrush colors at their full strength so I dilute the colors with vodka to at least a 1:1 ratio. It makes the color sheer enough that I can get the effects I want and also allows me to build color by spraying an area multiple times.
I also mix my colors ahead of time but I find that layering different colors adds a lot of dimension to a cookie so you might still want to consider that option.