>>4059496“Between Flat and Gradated Tones
What has been said about the balance of straight lines and curves applies equally well to tones, if for straight lines you substitute flat tones, and for curved lines gradated tones. The deeper, more permanent things find expression in the wider, flatter tones, while an excess of gradations makes for prettiness, if not for the gross roundnesses of vicious modelling.
Often when a picture is hopelessly out of gear and "mucked up," as they say in the studio, it can be got on the right road again by reducing it to a basis of flat tones, going over it and painting out the gradations, getting it back to a simpler equation from which the right road to completion can be more readily seen. Overmuch concern with the gradations of the smaller modelling is a very common reason of pictures and drawings getting out of gear. The less expenditure of tone values you can express your modelling with, the better, as a general rule. The balance in the finest work is usually on the side of flat tones rather than on the side of gradated tones. Work that errs on the side of gradations, like[…]”
Excerpt From: Harold Speed. “The Practice and Science of Drawing.” iBooks.
https://books.apple.com/us/book/the-practice-and-science-of-drawing/id492176539 
