In a negative picture, the colors turn opposite. Therefore, blue would represent something that should be yellow. Any yellow on the image would represent something that was supposed to be blue.
On Vision and Colors (German: Über das Sehn und die Farben) is a treatise by Arthur Schopenhauer that was published in May 1816 when the author was 28 years old. Schopenhauer had extensively discussed with Johann Wolfgang von Goethe about the poet's Theory of Colours of 1810, in the months around the turn of the years 1813 and 1814, and basically shared Goethe's views.
Schopenhauer tried to physiologically demonstrate that color is "specially modified activity of the retina." The initial basis for Schopenhauer's color theory comes from Goethe's chapter on physiological colors, which discusses three principal pairs of contrasting colors: red/green, orange/blue, and yellow/violet. This is in contrast to the customary emphasis on Newton's seven colors of the Newtonian spectrum. In accordance with Aristotle, Schopenhauer considered that colors arise by the mixture of shadowy, cloudy darkness with light. With white and black at each extreme of the scale, colors are arranged in a series according to the mathematical ratio between the proportions of light and darkness. Schopenhauer agreed with Goethe's claim that the eye tends toward a sum total that consists of a color plus its spectrum or afterimage. Schopenhauer arranged the colors so that the sum of any color and its complementary afterimage always equals unity. The complete activity of the retina produces white. When the activity of the retina is divided, the part of the retinal activity that is inactive and not stimulated into color can be seen as the ghostly complementary afterimage, which he and Goethe call a (physiological) spectrum.
Primary colors are sets of colors that can be combined to make a useful range of colors. For human applications, three primary colors are usually used, since human color vision is trichromatic.
For additive combination of colors, as in overlapping projected lights or in CRT displays, the primary colors normally used are red, green, and blue. For subtractive combination of colors, as in mixing of pigments or dyes, such as in printing, the primaries normally used are cyan, magenta, and yellow, though the set of red, yellow, blue is popular among artists. See RGB color model, CMYK color model, and RYB color model for more on these popular sets of primary colors.