Oh *facepalm*
It's my browser that isn't showing the colour correction properly. Bit of a Googling shows Chrome's colour management has been horrendously broken for the past half a year. Apparently it correctly convert any image with an embedded colour profile to the monitor profile, completely ignore HTML colours rendering them as is, and then completely ignore images without embedded profiles rather than assuming they are sRGB. The site strips out colour profiles on upload hence the results in the picture below: both images are the same default colour profile, but the large one renders correctly, the IIS one doesn't, and the side bar looks like someone's drawn it with a blue highlighter.
Ok nothing to see here folk move along now.
