I've searched high and low for an answer to this. What I find odd is that the only person on the internet who seems to have had a similar issue (or even recognised this as an issue) was one photographer (who referred to the colour as "magenta"). Of course, as a photographer he had access to colour calibration equipment and was able to solve his problem that way: a luxury I don't have.
What's puzzling is that no one else seems to recognise this as a problem. Other people's graphics card issues always seem much more serious i.e. nothing works, which isn't the case here. Yet it's the same for every Linux installation I do, with whatever distro and on two different computers.
The problem is that the colours are sickly, slightly blueish, anemic etc. It's "useable" and some non-photo images e.g. icons look ok but as soon as I view something with humans in it or lots of colour, it just looks wrong and it makes using Linux a chore. Even the grey of the windows in Gnome 3 and Linux Mint looks wrong (cold/alienating) and no amount of fiddling with xgamma will get it anywhere close to the quality of images in Windows 7. I've even tried to adjust the colours via xgamma with a PC running Windows 7 next to it, so I can use the same photo with the right colours as a reference. The same distro always looks fine in VirtualBox.
The main PC is using an ATI Radeon HD 5470 and the other is using something like an ATI R200 Express (no longer supported by ATI). However, even though a quick look at the internet reveals a huge quantity of ATI/Linux-related misery, none of it is of the kind I describe. The current distro is Linux Mint Debian. Previously I have had Ubuntu, Fedora 17 and Debian 6.0 installed, all with the same "sickly colours" problem.
Am I being naïve to expect the colours of a PC running Linux to be anywhere near those of the same PC running Windows 7?