system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How are photos colourised?
gpt
Some modern cameras store images much like these pictures. They use two channels, color and intensity. The intensity channel would be like the grayscale image. Intensity ranges from 0% (white) to 100% (black). The color is painted over this intensity, resulting in different shades of that color.

To colorize old pictures like this, an artist takes a relatively small color palate and applies it to the image, using the greyscale as the intensity channel. This way, the artist doesn't have to create shades of a color, the intensity channel does it automatically.

As for how accurate it is to the original color, it's often not very close. The artist may make a few educated guesses as to what color things should be (grass is green, sky is blue, etc.), but unless there is some other documentation as to what color the clothes, eyes, hair, etc., were, it's all just guesses and whatever the artist thinks looks good.