In the times when Rembrandt was painting, underpainting was common practice. It was the basis of starting a painting. It is still used today but for various reasons.
An underpainting in my oil and acrylic classes can provide important information to help boost the start of a painting. I teach my students to use this method as one way to start a painting. It will help them understand the setup their values, (lights to darks) and design and correct the drawing in paint before moving forward. This is done by using one color and white. Keep the ideas very simple, do not add detail at this stage, just get the basics of lights and darks set up on your picture plane. Then let this underpainting dry fully and build your full pallet on top. With this guidance on you canvas, it can be easier to then work on the complexities of color mixing and painting.
Here are an example of a blue and white demo and the same one with charcoal contour line on it.
In a more advanced understanding of painting, I did a charcoal on a yellow tinted canvas. Then I proceeded to do an underpainting in purple, which is essentially a complimentary start to a painting. I will let this dry and build my painting on top. These colors will enhance my greens and whites and make my painting sing with overlaying colors. This is not for a beginner painter.