If you want to be all smarty pants about it? You could refer to underpainting as Grisaille.
You could also point out that I am a watercolorist (slash) mixed media artist. Therefore, what I am doing could not be, is not, truly Grisaille.
I get it.
Grisaille is an actual painting style that never involved watercolors (or paper come to think of it), if you're looking at it from an historical, purist perspective. Some people have loosened up in modern times by defining it as painting in shades or tints of a single color.
So I use a technique I comfortably refer to as underpainting.
I do my pencil sketch, and then begin to add very light watercolor washes to my paper.
I do this because I like to map out what I am about to do. I block in colors, shadows, and leave highlights untouched. I figure out wing and feather patterns by kind of blocking them in. With mammals it allows me to figure out the direction the fur lays over the curves of the body while still preserving the lightest lights and darkest darks.
So, in the top photo, do you see that section on the eagle's back and shoulder? I didn't touch the paper there. Now look at the photo underneath. I knew that if I wanted that yellow/gold to actually look vibrant and golden and not like mud (therefore killing any depth perception between the wing span) then I needed to remember to not touch it during my painting process.
There are other benefits in using this technique!
I have found that it helps to fill in the little divots on textured paper. It provides a nice surface on which I can then layer and build and intensify color richness. And, honestly, once I've sort of planned it all out, I can then enjoy painting with a bit more confidence.
Underpainting also takes care of that Fear of the Blank Page Stage that keeps so many people from beginning at all! A nice light wash breaks the ice, and you now have created a place to move color value up and down.
I know the watery-ness of watercolors feels out of control, flow-y and permanent to a lot of people who are just starting to use it. And you're not completely wrong. It is very easy to add too much color resulting in dark, muddy pictures that ruin a fresh, light vibrancy to the pigments. Or too much water and scrubbing begins to ruin and lift the surface of the paper.
It also depends on your style. I obviously control my watercolors and use them kind of "dry".
With this medium there's not a whole lot of going back once those things happen.
But with some tips and tricks and trial and error you can gain confidence and control over what you want to achieve. Underpainting is only one of many ways to go about tackling design and technique. No matter what your style is.
Give it a try if you don't already do something similar.
It may just boost your confidence by offering a bit of control with your watercolors!