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Here is an example of how using a burnt sienna undertone can help to create an overall warm feeling to a painting. There are several benefits to using a tone to begin with on the canvas. First of all it eliminates having to deal with all that white right off the bat which is sometimes overwhelming when starting out. Secondly, depending on the painting, some of the underpainting can be left as is with a slight scumble of color over the top of it leaving two layers of color for the price of one. This is especially helpful if using oils especially if the undertone is dry before the painting is started.
This warm tone can even be helpful in areas such as the sky to maintain harmony within a painting. Starting out with a middle tone already on the canvas lets you concentrate on the darks and lights while placing the values in the piece and again eliminates that middle step.
I like to use burnt sienna but depending on the painting, other colors can be used just as effectively depending on the mood you'd like to create.