Gouache is a tried and true media that illustrators have been using for generations. It is remarkably fast, revisable, and portable medium, making it wonderful for life sketching.
In this 2+ hour tutorial, Anthony Palumbo will walk you through his process of creating fast, beautiful sketches using a combination of watercolor and gouache.
Get the video right here:
https://www.muddycolors.com/product/sketching-in-gouache/
Rented it and devoured it immediatly! Good stuff!!!