Colour differences between 2 Dell Ultrasharp monitors

I've just bought two 23 inch Dell Ultrasharp U2311H LCD screens (IPS). As soon as I switched them on, I noticed that are significant colour differences between the two screens; the left screen felt too cold / blue. I tried adjusting the RGB values manually but haven't been able to fix the problem.

To verify it's not a dodgy output from the graphics card, I unplugged the video cables and ran the diagnostics screens (displays grey, red, green, blue, white and black screens without input from the graphics card). No funny pixels, but the colour difference between the screens is very obvious, especially on the grey screen (RGB seem fine).

I photographed the screens in their grey state, see here (2 MB):
http://fileshar.es/Te9f2J5

Do I have a defective screen? I have no calibration equipment, but the colours are obviously well off on the left screen.

Can I claim this on warranty or is this sort of variation normal? I've been using dual screens for years, albeit lower quality ones, but have never had a significant colour variation between any of them.

Thanks!