I hate beige/custard colours with a passion but it's understandable that people would pick neutrals(greys/whites) for their rooms as it means they can completely change the look of the room by adding accent wall colours and accessories. Buy one nice sofa in a stone colour or something and then put a neon pink throw over it and it's a different sofa. But then if you get bored of neon pink you just switch it out. Same with rugs and wall hangings. Can't imagine a lot of people have the money kicking about to fully redecorate every few years and even more might have strict guidelines from their landlords and how they're allowed to paint their flats.
Another thing to consider is that any carpet past a certain darkness will show every tiny bit of dirt(?) on it. I needed new carpet in my room as a teenager and somehow during the height of my emo phase I convinced my parents to get me blood red. It was so awful. You could vacuum the whole room and 10 minutes later it would somehow have little specks all over it again. It never looked vacuumed.
Most rooms in UK homes are quite small and some are little more than closets, you can't paint them deep bright colours unless you want to make them look even smaller. White/light walls lets light bounce about -another issue with our terminally small windows. You can get away with it easier in a Victorian because they high ceilings. Growing up, we moved to a Victorian with several floors and the previous owners had painted with entire hallway system 'B&Q' orange. I love orange and bright yellows but it was just so much. It made everything else orange and the hallways always looked darker and murkier(?) before we repainted them a purpley-grey which looks more like matte silver. It went a lot better with the white bordering as well.
I assume the car thing is to make them easier to sell on. Cars are poor investments as is if you're thinking of returns. I do think it'd be nice to have some more refined colours available though like emerald and other autumnal colours which aren't too loud. One advantage of a million nondescript silver cars is that if you have a stalker, they're much less likely to spot your car outside your house. Bit of a weird observation but my mind always comes back to it.
I'm surprised you didn't mention in fashion. Next time you're out, have a look at how many people are wearing colour, best you'll get is blue jeans.