If it is supposed to look like the picture then spray the color fades first and then lay down chicken wire, fish net, laundry bag or anything that gives you the desired pattern and spray the black. It will com out excellent. I do it all the time on other types of projects to get patterns.
EDIT -- I looked at the guitar pic an it is different than the photoshop.
Spray gold and then lay down the pattern material and spray the other colors to make your fades. Not very hard as you will find out. Hope you are using a spray gun and not cans though. Also spray straight down into the pattern.
You could get a mask made to do this with relative ease. I cut stuff like this on my plotter all the time also for special effects that require a stuck mask.