You have to consider the calibration of your monitor, the colour profile embedded in the image (sRGB versus AdobeRGB versus no profile at all) and the web browser used to display the image. Only Firefox reliably interprets colour profiles properly, closely followed by MS Edge. Chrome is hopeless!
As RB points out, always use the sRGB colour space for images to be displayed on the web.
In any case, step one is usually to calibrate your monitor.
Cheers, Marcus
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