How people get to know which colour it is when they convert a black and white photo into coloured one?
In: Technology
It is generally research and informed guesswork.
If you have the original negative and know what kind of black and white film it was then you can check against a colour response curve for that film which will give you some clues as to what a particular grey is likely to be – but this still involves a lot of guessing. Mainly it is by consulting other references, e.g. we know that leaves are green, certain street signs are red or blue, bricks are red, slates are grey etc.
There is one exception to this. If you have a black and white film of a video that was originally photographed in colour then there is a recently developed process that can restore the colour based on the existence of “chroma dots” in the black and white image. This technology is being used to [restore the colour to old TV shows](https://www.theguardian.com/technology/2008/dec/11/digital-video-restoration-dad-s-army).
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