Arrrggh I knew someone would ask!
I only know about it as we sold an aircraft a couple of years back which had extra stickers/decals applied and the company agent removed them as they were in breach of the regs because there was more than ~10% colour added. (or whatever the figure was)
I do specifically recall when CASA took an operator up north to task that their aircraft were fitted with decals that weren't approved by CASA. In my defence, perhaps this was the materials that they were made from that didn't comply?