Does anyone know how to prevent this? It doesn't lose as much colour if I just screenshot it. But even the screenshot on the left is still slightly more muted than the original photo. I appreciate you taking time to respond. If you can't or don't know what this is, that's okay too.
Looks like the color space is being read improperly somewhere.
Not sure if this will fix your problem, but you can set the color space of an image with the ColorSync Utility (should be preinstalled on macos). Open the image (File > Open), and at the bottom of the window select Apply Profile. Try experimenting with profiles in the second drop down box to see if the problem goes away (they shouldn't change the way the image looks on your screen).
sRGB should be compatible with everything, but it's old and doesn't have a very wide gamut. For your image it should be fine though.