When I took marketing in college pretty much every expert blamed the brownification of McDonald's and other restaurants on the success of Starbucks. Bright colors encourage people to keep moving, earth tones encourage people to relax. Starbucks showed the value of making your restaurants into places people could meet up and hang out at instead of just a stop to get food, especially relevant with the proliferation of wifi devices, and the fact that the customer base was self-segregating with the people who wanted to eat and run increasingly choosing the drive through. Lots of restaurants, although this is far from universal because it's expensive, now have a table or two with more comfy chairs instead of the traditional booths and metal ones, following the logic through to its conclusion.

Now we're seeing cultural pushback on the brown, if it keeps picking up steam we'll see a response from McDonald's corporate, and the cycle will proceed to its next phase of appealing to millennial nostalgia. But the logic of "make people want to stay in the restaurant" will still be the same.