White doesn’t fade.
An average airliner will have several paint jobs during its service life, but the longer you can go between having to repaint the plane the better, and if you don’t have to worry about the paint fading and looking old for a good long while you can stretch the time between paint jobs without having your passengers start making snide comments about your fleet of flying hoopties.
All colored paints will eventually fade from exposure to sun and the elements, particularly exposure to the sun at 30,000 feet, where substantially more UV radiation is hitting the paint and accelerating the fading process.
White looks better and when it gets old and dull, it still looks good compared to a dark colored plane. Dark colors fade faster, dull, and when paint flakes off it just looks terrible.
Decorative painting are expensive
Cost can run anywhere from $50K to $200K per plane. The 2-3 weeks of time required for such paint jobs will cause loss of revenue for that period.
More paint= More weight= More fuel = More operating costs
The paint on a fully painted 747 will weigh more than 250 kgs, while a polished skin will just weigh 25 kgs. EasyJet Airlines was able to reduce 2% of their operating costs by just repainting their planes with new thinner aerodynamic paints. When your annual fuel bill is $1.2 billion USD, then that 2% can mean a savings of $22.4 million USD. Suddenly a coating becomes interesting, doesn’t it?