Blue truck nuts means he hasn't had any for a while.
Brown and Black truck nuts remind people of the ethnicity of the driver.
Pink truck nuts are for girls that arn't to be messed with, or just like you to know that their car is a boy.
Brass Truck nuts... well you know "he's got brass balls".
Red or blue colors are used to indicate if the vehicle owner is a "top" or a "bottom" while other colors indicate "no preference" as well as usual, or unusual preferences.
Size of the nuts have no direct relationship to the actual size of the owner's penis and are usually misleading.