772ed185_Grand_Theft_Wiki__the_GTA_wiki__Description
[Ability] Unique Paintjob [Code] Color With Bold Print [Description] 
These are simply cars with a unique paintjob to it, that can't be obtained via Pay N' Spray or mod shop, and can only be found in the specific missions they are created for. Using a Pay N' Spray on any of these vehicles will remove the colour and it won't be obtainable again.
Special Vehicles - Grand Theft Wiki, the GTA wiki