I think it's probably a fairy tale, like us saying it was so fishermen could see their homes through the fog.
That said, we do have
very grey winters. Our weathermen and women often use the phrase "RDF" meaning "Rain, Drizzle, and Fog". It's overcast, foggy, and you need your car wipers on the lowest speed - but standing around outside, you don't really get wet. On average, however, we do get more hours of sunshine than other famously grey cities, such as London.
I suspect the answer is probably something much less romantic. Coloured paints were more expensive than whitewash and a luxury most poor families wouldn't have been willing to spend more on - so it became associated with the wealthy and, therefore, desirable.
I see the same things in myself. Right now my house has a very plain front (it's the one with the orange door):
But my father is already building door, window and roof details to fix it up a little, and make it look more like a wealthier home, such as these upper-middle class examples:
So, to me, fairy tale reasons why we use coloured paint are about as realistic as saying, "SignalHillHiker wants wooden details over his windows so that he doesn't get depressed in the winter, or so that he can see his house from the hill."
I think the answer is simply... these things were luxuries the poor couldn't afford or weren't willing to spend money on. Now we can, so we want them too. They're like having a better model of the same car.