Flat Color illustration is my favorite pass-time. My love for this style started with a project from my Pre-press Production course in college. The far left image in the row below is from said project; we were asked to design all six sides of four cubes based on a four letter word of our choice. I chose "Depp" and illustrated four of Johnny Depp's best characters in a flat color illustrative style for one side of the cubes.
From there, I went on to illustrate Dodie (a youtuber and musician), and a friend of mine who asked for a portrait—see the middle three images below. Those three illustrations led to two smaller musicians reaching out for portraits to be used on their tour posters—see the final two images below. Every-so-often I do still receive commissions for flat color portrait illustrations or I seek out images to illustrate in this fashion on my own too keep up my skills.