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Take a sample to a shop, have them do the colour match and mix.
Doing it from a photo guarantees that they match the colour as per your screen, and it won't quite be the same.
If you're colour matching then Johnson's is the best paint (so says painters I use)... ignore the posh paints, colour matching is #1 Johnson's, #2 Crown, #3 Dulux... and there's not much between those.
Now... a wall... that does make it hard. You wouldn't consider just giving that wall another coat so that it matches all the rest perfectly?
How good is colour matching paint and how do you do it? (I'm assuming that a photo is going to depend on the camera, lighting, etc) Would like to colour match an existing wall.