E.g. you can enhance the light-dark contrast by drawing the darkest areas darker and leaving the brightest parts white. That can put a completely different dynamic to a pichture and everything appears more plastic. In addition you can influence the level of realism of a picture through the quality of the shadows you draw. When you are drawing the gradient more precisely, the whole picture will apear more realistic - if necessary photorealistic.
(As an example for an realistic drawing, you can take a look at my picture "Horse and Rider" in the Picture Gallery. I would say that the image is drawn very precisely, so that you can hardly see a pencil stroke, so it come very close to photo realism. However, whether such a realistic drawing is desirable remains to the own taste. Photo-realistic drawings look very impressive, but often they own less character then a simple sketch, which has the artists typical handwriting.)
With the colored pencils there are even several different methods of drawing a black-and-white gradient:
The simplest method of drawing a shadow gradient is to draw it with only a black colored pencil. To draw the shadow you just have to put a lot of preasure on the pencil in the darkest areas in order to achieve a strong color application. Then you just have to reduce the preasure in the brighter areas till you finish in the gradient in the white. With small strokes the shadow will become finer and better, as if it would be drawn quickly with big strokes.
Now you can recognize the structure of the paper in the gray areas as small white spots. The rougher the paper is, the more structure you can see. This effect is not necessarily unwelcome, in most of my drawings however I eliminate it. How to eliminate the paper structure in you shadow drawing, I will describe under point b), c) and d).
The already mentioned eraser pen should have two different eraser tips. One with a hard eraser (mostly white) and one with a softer eraser (mostly pink). The hard end is suitable to clear away details from a finished drawing, eg for highlights. The softer end can be used to blur the color on the order paper.
A simple gradient (as described above) can be softend with the help of a eraser pen by moving the eraser penthe in a circular motion over the drawing. Put only a few preasure on the eraser pen when you are doing that. Thus the color will drown deeper into the pores of the paper and the effect desribed above ( (with the small white spots) will be eliminated mostly.
A better method to make a good shadow gradient is using additionally a white crayon. A white crayon is doning basically the same as the eraser pen, you just have to put more preasure on it. But if the gradient was drawn sloppy in advance, you'll be in danger that the gradient will look uneven, after you gone over it with the white crayon. So if you put emphasis on a very smooth gradient, you should take care allready from the beginning. Best results will be achieved by using a sharp colored pencil, because the colore will get deeper into the structure of the paper and therefore less white spots will remain. Another option is to rework the gradient with an eraser pen and then with a white crayon (hint: before working with the white pencil, remove the eraser residues; e.g. with a small brush - don't use you finders as they are greasy).
color gradient arranged with a white colored pencil
An more elaborat method of drawing a black-white gradient is using colored pencils in various shades of gray. For a simple gradient the white and the black pen, plus one to three differnent gray pencils will be enough. It all depends on how far the gradient will go. If you intend to draw e.g. a shadow gradient over an entire A4 paper you should use a lot (e.g. 8) different gray pencils. Drawing the gradient with only a black and a white pencil would be - let's say - a challenge. But ordinary a few gray pencils should be enough.
This website is about how to draw and paint. You will learn to use pencil and brush and how to drawn and paint your own pictures.