TriColVid (Trichromatic Colorization of Video or 'TCV') is a project of mine with the aim of developing accessible and on-demand colorization of films, in particular historic black and white films. My project also aims to demystify some aspects of color vision and use that to explain why accurate colorization is (still) very challenging.
The 'trichromatic' alludes to the fact that we humans perceive three colors of light as primary colors; these are red, green, and blue. Our eyes 'break' down the light that enter them into these primary colors, which are processed by cells called cones. All other colors of light, except black, are mixtures of red/green/blue lights in varying proportions depending on the color we are 'seeing'.
We 'see' the color of an object or body because external light has shone on it, some of which the object/body might have absorbed and the rest reflected (to our eyes). It is what is reflected to us that we perceive as the color of that object or body. For instance, leaves are green because they absorb red and blue lights but reflect green. Banana skins are yellow because they absorb blue light but reflect red and green in equal proportions. Oranges too absorb blue light and reflect red and green, but not in equal proportions. A white object reflects all three primary colors equally and a black object absorbs all three.
What we call 'black and white' images are technically called grayscale. Trying to identify the original color from a particular shade of gray in a black and white image is by no means trivial. I elaborate more on this here.