Oh boy I hope I did this right. Since Anon asked, here’s a hasty tutorial on how I do flat colors. This is also how I color pretty much everything in my webcomic. Now you know my secrets!
Edit: WOW this has an excessive number of notes. Also, this method doesn’t work very well on areas of really light colors, like super-bright pastels and white— the shadows wind up really really dark and the gradients have very little effect.