When you take a B\W photo, how do you get the main subject of a photo to be in Red or whatever colour, with the rest of the photo B\W?
If you mean like this...
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That's called "Selective Color". It can be done in-camera, but I think most people probably do it in post processing. To do this in Photoshop I create a B&W layer mask and then use the Brush tool to "remove" the B&W white layer selectively which then reveals the color layer beneath it. Really simple.
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That's the thing about PS, there's probably several ways to do just about anything.Gonna have to try it like that Fish.
I usually just select the area I want in color, then do an inverse and do it up B&W .