In my starting sketch I define the composition and the positions of the characters. I give each one a different shape and personality so the scene will be the most interesting possible and tell a story, whether in a direct or indirect way. In the process, I usually refine the characters. I keep in mind that everything I do at the beginning can be improved or totally changed by the end.
First element colours
I start painting one element or character first so that I have a reference point for the rest of the characters and the scene as a whole. I think about whether the colour palette will be hot or cold and more saturated or less, and where the light source will be placed. I always imagine this first element with a white and neutral light and then mentally make it colder or warmer.