The list of colors is finite and based on EU law[1][2], so it would be odd if Greece didn't share the same system. It would mean you couldn't import re-painted vehicles into Greece, maybe that's true, but that seems like it wouldn't fly with the single market.
The car in the article clearly has a base color of "yellow". On my street there's a car completely covered in stickers, to my eye it's mostly white, but looking it up I see it's registered as blue (barely any blue visible). So perhaps covering it in stickers is a way to get around this in some areas of the EU.
1. https://www.rdw.nl/particulier/voertuigen/auto/het-kentekenb...
2. It's: blue, brown, yellow, gray, green, orange, violet, red, white, black. Anything not listed must be whatever color is closest, e.g. pink is either "white" or "red" depending on if it's light pink or not.