Customization Doubler doubles the amount of paint textures and colors you get from each Customization Pack you own. It also increases each pack's effect on the maximum metallicness and smoothness. It affects both the packs you already own and the ones you might get in the future.

Without a Customization Doubler, each normal Customization Pack gives you access to at least 20 new paint textures and 4 new colors to be used in paint jobs. With the Doubler, the numbers are doubled to be at least 40 and 8.

Of course, this applies to Customization Bundles and 4-packs, too. As you get 4 packs from each bundle and 4-pack, Doubler changes the minimum amount of unlocked paint textures for each from 80 to 160, and new colors from 16 to 32.

NOTE: You must own at least one Customization Pack or Bundle for the Doubler to have an immediate effect. If you don't own any, you'll need to acquire some to benefit from the Doubler. Also, Doubler doesn't increase the amount of paint commands available for each paint job, just the number of available colors and textures.