That's interesting... actually the directive only specifies that the numbers should have sufficient contrast against the background, be at least 10 cm large and have their own light source. So, while most seem to have settled on "black numbers on a white square", you are actually allowed to do something fancier if you want. Other cities (like Munich, which I mentioned in a sibling comment https://stadt.muenchen.de/rathaus/stadtrecht/vorschrift/310....) have more strict design rules, but don't require illuminated numbers.