How to capture a scene on location in a city. What should you focus on at the location, and what can you do later (to tweak from a photo)? Maximize your time when in an urban setting. Pace your art to prioritize color dots above all else. Squint to simplify. Paint with a friend if possible, there is less chance of people commenting and someone else can 'watch your back'. Mostly I found everyone to be very supportive of art getting created.
City Painting Tips:
LOGISTICS - Try to set up on location in a city or bustling town. It's hard to get motivated and believe you can actually do it. But you can! Tuck out of the way of the main walk ways and paint with a friend when in the city. If sound really disturbs you try earbuds (even if they are off they mute the sound a bit and send the message you are working).
PERMITS - Some cities in busy areas (and parks too) have permits required for anything that looks like a money-making setup, or a busker. Just be polite and move on if asked by an officer. When painting in Paris we did not encounter this, but I've been asked in other places if I have a permit (I do sometimes). Usually it's more of a problem for a large organized group, and especially if it's for a class or if you are setting up a pop-up display of art. Painting without an easel is usually fine, or sketching at a cafe.