BPelt’s Multifill and Flatten Pro tools can automatically add flat colors to your illustration with but a few clicks.
I whipped this up real quick because I’ve tried to share this tool with several artists and most of them don’t seem to comprehend how awesome it is. Seriously, it’s really awesome. Especially if you’re paying people to flat stuff for you.
Of course, there are ways to make these tools most effective, so I’ve detailed what I’ve learned so far. I hope this helps!