I’d love to do something similar, but the Greek law doesn’t allow it. The car registration must clearly state the manufacturer color(s). Try describing/putting the graffiti into this field :-) We’re not even allowed custom license plates.
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.
1. https://www.youtube.com/watch?v=HkhFnQtJZls
There must be an "other" option, or at least, everyone who has to use the database will soon learn that when the model is "Harlequin", "yellow" doesn't quite mean "yellow".