While “saving for web” in Photoshop, if you notice a desaturation of your colors, here is a quick fix:
Before saving for web, go to “Edit”, “Convert To Profile: sRGB IEC61966-2.1”.
This happens because browsers aren’t color-managed, so you’re seeing the results of Photoshop stripping the image of its color profile.
You can read more about this issue here & can set your monitor to this workflow.
Note: prepress users will not want to recalibrate their monitor to this setting permanently, but can instead use the quick-fix tip above.