That always depends on the color you are spraying..some colors will even come with a tech sheet that says the best option for a certain color primer for easier/better color coverage.
For example,I used a VW Metallic Reggata Blue for a project and it said to specificaly use a black primer to acheive the best shade and coverage it was intended to be.
But as far as camo colors,flat/semi colors..etc or just an enamel or such..grey is usualy the most used since it's the most common available.....
Black or grey will work on just about anything,unless your looking for a specific exact match of a color your shooting for...
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