system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How to convert black and white photos to colour

How does it work? Do the black and white shades have coloured counter parts?
gpt
You colour it in by hand (usually in photoshop) by adding colour on top of the black and white shading.

It's like painting over a pencil sketch with watercolour paint, you still see the pencil shading underneath but the watercolour adds colour on top.

To get it looking realistic you sometimes have to lighten or darken bits too as some colours in B&W photos are too light or dark, and the whole thing takes time and skill to do right.