The farther away an object is from you, the more the variegated reflections of white light from a distance will cover up the colors reflected from smaller objects and shadows. An additional factor is reflection from the atmosphere itself.
For example, if you gaze into the entrance to a tunnel, the darkness at the entrance will lessen in intensity the farther you go from it. The air between you and the tunnel reflects white light, and the greater the humidity, the more light is reflected.
To the human eye, the absorption of red and yellow rays seems to be correspondingly greater when blue rays are reflected. Thus it happens that in humid conditions landscapes appear bluer and more spacious than they do in a dry atmosphere and sunshine: clear colors come forward toward the onlooker. This is a phenomenon which a landscape painter must fully appreciate if he is to depict what he perceives just as he perceives it.
The Chinese were masters of the art of conveying a sense of space by suggesting a darkening atmosphere, particularly in graphic work painted in thinned China ink, with a near color effect. In classical Chinese art, linear perspective played a very minor role, if any.
Alongside this atmospheric darkening, the plasticity of silk, which reflects the light in tiny particles, also serves to achieve depth of perspective. Coarse canvas can be used in the same way. Pictures executed on canvas whose texture remains visible have more atmosphere to start with than those painted on a smooth support. This also applies to watercolors on rough paper. Where the texture of the support has its own light and shade, the picture not only gains a surface, but also produces an impression of several planes graduated in depth. Moreover, the texture helps to some extent to avoid crude and blotchy shadows which look like holes.
The Impressionists were the first artists in the western world to "see" the atmosphere overcast with all shades from gray to white in a bright light. They achieved an effect of depth and flatness simultaneously in their pictures by the appropriate mixture of grays and whites in their colors. The plastic thickness of the paint also helped, resembling as it did a woven fabric. Here the effect is even more forceful and conspicuous, however.
The Italian artists of the Renaissance had already discovered and rendered a shadowy atmosphere instinct with mystery (you will recall the comparison with the entrance to a tunnel). This was the celebrated sfumato, the smokiness, quite different from atmospheric humidity. They saw it not only in open landscapes, but indoors too, in the rooms where they painted their portraits and figures.
This sfumato enabled them to banish from their pictures the harsh contours which had previously marred the pictorial effect. Over the main parts of the painting, they set the light flowing forth in a gentle glow against the dusty obscurity. Analogous aims and effects are attained in photography by placing a soft-focus attachment in front of the lens. Leonardo da Vinci and Titian were exceedingly enthusiastic about the phenomenon of sfumato, but they used it with the subdued grace of their day. They were, of course, among its most glorious exponents.
Then came Rembrandt, who shattered this sobriety, carrying the trend relentlessly to its utmost limits. He rendered darkness and shadows with an intensity to be found nowhere else in the history of painting: and yet he did not create "holes." The black of the The Man with the Gold Helmet is shot through with a riot of color; the air becomes palpable, suggesting that one is feeling one's way through a room utterly devoid of light.
One senses vaguely what is concealed in the shadows. Rembrandt produced this effect by using countless glazes. He is understood to have used 30 to 60 of them, and Titian is known to have worked in the same way. In all probability the glazes were carried out in complementary colors, and these produce shadows which no longer appear to be colored. How different from the workaday black which stares up at you from a tin of shoe polish!
The discovery of the visible atmosphere and the first attempts to use sfumato in painting it mark the beginning of something which can properly be called "painting." Until then, artists had colored rather than painted; a single object was rendered in local color and distorted in the process. Color perspective was used only by accident, not consciously. Linear perspective was almost the only means employed to give an impression of depth.
To penetrate the mysteries of color perspective, while bearing in mind that the ever-changing exceptions and special cases far outweigh strict rules such as those of linear perspective, you can hardly do better that follow the main stages of its development; after leonardo and Rembrandt, Watteau marks just such a stage before the Impressionists, and among present-day artists, Kokoschka has given it fresh significance.
The alpha and omega of the study of color perspective lies in the perpetual observation of nature. This will reveal that when colors are blurred for purposes of perspective, the result is not neutral gray, but a variation of tints on the original color, accompanied by a blurring of outline.
The tinting and brightening of the original color (or its darkening, for that matter) is liable to alter at any moment. For example, red does not always turn to gray-pink; the nearer the color is to the horizon, the more dependent it becomes on light conditions. You may have occasion to observe a tiled roof in widely differing circumstances: in the sunlight, rain, and under a cloudy sky, in the morning and in the evening; each time it will look completely different; and to paint what you see, on each occasion, you would need every color you have.