It does work, but it's grey because it's not finding the right file. Unfortunately the naming conventions are not entirely consistent, so you need to make sure you have the right file name for the custom liveries. You can check the .rcf file for what the file prefix needs to be. For example, the DTM car is expecting MERC_DTM13_LIVERY_PLAYER.dds instead of the full name for the vehicle like most cars (similar to how the stockcar_90 files need to be named STOCK90). I thought I had put a message in to warn people of this, but I added the support for the additional cars at the last minute, and this debug message might not have made the build.
Added to the list to check on this debug message.
I have the Hires/Lowres livery issue sorted out already, as well as dealing with different vehicle liveries with the same folder name. I'll probably wait through the weekend to see what other issues crop up, then release a hotfix next week so that people don't have to re-download all 1.5GB again.
EDIT - Here are the names for the newly paintable cars (case-insensitive):
AUDI_90
AUDI_R8_LMP900
audi_r18
audi_r18_etron
bentley_speed8
mclaren_f1_gtr
MCL_MP4_GT3
merc_dtm13
toyota_ts040
So any liveries created should have at least the file (for example) AUDI_R8_LMP900_LIVERY_PLAYER.dds included.