Once you start feeling like you're getting the essentials of a figure down, it's time to add color. If you're short of time or somewhere you can't whip out a small watercolor set (Buy Direct) and waterbrush (Buy Direct), add some written notes about colors and tones.
Then, when you get back home or have more time, you can paint in the colors from your notes or simply from memory. If you can't remember the colors, use whatever is plausible. Keep it simple to start with, don't bother to try to color match exactly. Rather aim to match tone, for instance is the person's top lighter than the trousers, are their coat and shoes light or dark? Again, with practice, the speed at which you observe and then record what you've seen will increase, and you'll get more and more color down, as well as become more focused on tone.
