From what I understand, it is a money issue. New paint jobs on aircraft are being done at a constant rate. And sometimes the airline can save money by designing a new paint job using more cheaper paint colors.
One classic example that I know about is NWA'a new paint scheme. The red color that was very dominate on the top 3rd of the aircraft was one of the most expensive paint colors. So then they design a paint scheme that has less red, and each A/C paint job is then cheaper.
So, looking at the UAL new paint, I can assume that the grey and dark blue were a spendy paint color, so now they too are going white dominate.
Side note...NWA's new scheme, I think, looks pretty sharp, especially up against a white, snowy ground. EXCEPT it drives me crazy how the logo on the right side of the tail is pointing to the wrong cardinal direction. (NE) Any comments?