I always gesso the canvas first to make sure the surface is an even tone. Then I drop the opacity down of my preferred underpainting color with a medium. For oil I use linseed, but acrylics have a medium (that escapes me of course). :P
My teachers always taught me that if you plan on doing a warm painting you should prime with a cool color to balance and vise versa. Hope that's a helpful hint.
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