I've not read all 160+ replies as yet, but here's my input:
It's a city, a whole city, and we have seen a very small portion of it during a limited part of the day. There are 6 confirmed districs, each unique with their own style and inhabitants. What we have seen is a residential area, which would be designed to be an advertising playground, because the majority of the citys consumers are going to be clustered there. So it is going to be bright, full of advertising, lights and sounds - and vending machines, theres tons of them in the gameplay.
The people are varied and mostly colourful - punks, rastas, gangers executives, fashion junkies and homeless people. All types and walks of life were on that street, do we want them to look like an army of Agent Smith's? Hell no!
The Matrix was careful to not use too much colour, because of how desperate the situation was, it was a post apocalyptic cyberpunk setting. It worked for the movie. Fallout uses colour in a similar way, again, post apocalyptic. Blade Runner used a lot of bright colours and bright lights, but the rain made a contrast, washing out the colour and detracting from the brightness. Ghost in the Shell and Altered carbon do this the same way. Appleseed is almost entirely in daylight in a bright and clean city, and that suits its purpose, bceause the city is supposed to be an Eden.
Also, it's California! How is it not going to be sunny?
In short, the colour and brightness will be used to show contrast between area's of the city and the people that live there. I am more than sure of this. And it will work amazingly, I have confidence in that.