Convert To Grayscale

I was attempting to recolor a sprite, and I realized the best way would be to convert the sprite to a grayscale image, add a new transparent layer of a solid color over it (resulting in a sprite of the new color with correct shading), and tweak it until it looked good. However, I did not find any mention of any similar features in Pyxel.
I would like the ability to convert an image or selection to grayscale (black/white).