Colorful Airplanes have low resale value.
Painting an aircraft in a color other than white can negatively affect its resale value as the buyer will have to repaint the plane which will further add more weight to the plane.
The airliners which purchase brand new planes instead of leasing are almost always purchasing by using huge long term loans. This is a huge investment in a competitive market. They may themselves want to sell or lease their planes if things don’t go well.