What's the name of the method of sketching a head with a base color and then sketching it?

There is no such drawing method, at least it is not a common method

I think you are talking about Pencil Tempera

Pencil (or pen) sketches on the basis of thinly applied light color for practice, this drawing method is called "pencil tempera"

Theoretically, this is the scope of the work of art in recent years, but the formation of a unique style

The Miyazaki Hayao "Totoro" in the picture below is this type of childish feeling leaping off the page.

This is the type of childishness that is seen in Hayao Miyazaki's Totoro, pictured below

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This picture was found on the Internet, and there is no good head painting

Something like this

This kind of watercolor doesn't rely on light and shadow and color blocking alone, but mainly on outlining

First, outlining, then laying down the color, and later, depending on the situation, you can make up the line

Note that the pencil line is better if you can see it, and it doesn't need to be too engraved. The line is too heavy, too complete, too clear.