When I color tiles using the negative space method (where I fill in around the shape rather than starting with the shape and coloring around it), I start with a layer of underglaze covering the whole tile. Usually I choose the Velvet underglaze by Amaco, Jet Black.
For this tile I did something different. I colored half the tile in chocolate brown and the other half in Medium blue. Then I set to work with my additional colors to create the images.
Clay tile, Velvet underglazes on lowfire clay, fired at cone 06, 7″ x 7″, 2019.