LunaC - 7:49 am on May 4, 2013 (gmt 0)
At first I thought it looked the same in Safari, then a second later it didn't. Turned out it was the viewing angle (from above at first, sitting on the floor with the macbook on floor in front of me, then later face directly in front of screen).
It's not an answer to why they're different (both are saved in RGB, no profile), but it might be why some see the colour shift and others don't.
In Firefox and Opera, no matter what viewing angle, they matched up so it's a bit of a mystery.
Try what Lucy24 mentioned, save as png and see if that works better. Another way to go is to use CSS for that box, it can give the rounded corners and slight gradient and might have fewer issues, will load faster to.
Hmm, after staring at that for a bit I now have a huge craving for pumpkin pie. Mmm, pie. ;)