Color inversion is a photo effect that flips all colors to their opposite hue on the color wheel. For example, white becomes black, green becomes magenta, and blue becomes orange.
The interest in color-inverted photography started with analog photography and film negatives.
Chemicals in the film interact with the light that passes through the camera and darken the film, turning it into a photo negative. It takes several rounds of photographic processing to lighten the colors to their natural look. This is why photo negatives appear dark.
Negative film is not exactly the same as color inversion. For example, in film negatives, dark areas are light, light areas are dark, and the whole image is a sort of dark orange.
In a color inversion, all colors are reversed to their complementary colors so that even in the reversed image, you can still have bright, vivid colors.
Despite these differences, many people use the terms negative effect and color inversion interchangeably.