The skin called "carname_DEFAULT.jpg" is the AI skin file (it's also the default player skin). You can make the AI use whatever skin you want, just rename it to "carname_DEFAULT.jpg" and that's that. Rename the default skin or keep a copy somewhere in case you want to use it again.
Note: all AI cars use that one default skin, so you might want to use one that's sort of neutral in colouring due to variations in the AI colours (there may be a way to adjust the AI colours. If there is I'm not aware of it).
Similarly, when you race online, you will see the default skin on your opponents' cars (unless you have their skin files).