Fun with QR codes - what colours can you print them in?

I'm toying with print colour options (one colour black on white background, or red on green, green on black etc) and wondered what the tolerance of QR codes might be. This isn't very refined as I'm playing with the most basic picture editing software (Paint) which is coincidentally the most advanced I know how to use (I do this sort of thing so rarely I just play with mockups and pass on print jobs to our design team).

In case anyone else is interested in the contrast tolerance it seems that two colour QR codes don't work very well (I've tried a few other colours in addition to the ones here, might try very pale grey for completism!). The ones with a yellow oval next to it worked when using ScanLife for iPhone - it takes you to our homepage at http://www.chi-med.ac.uk

Possibly if the green and white one on the bottom right had a boundary around it that would help, dunno.

My advice is stick with black and white where possible ;)

Mildly tempted to swap bits around and see which bits of the image are essential and which can be tweaked with no loss of function.

This QR code below works fine - it has a white boundary and fairly good contrast (assume it's sufficient to be machine readable) although I'm sure I'd be confused by the image behind it.

This image is by Wikiwatcher1 who has posted it to Wikimedia Commons under a creative commons license - the original file details are here http://en.wikipedia.org/wiki/File:Wikipedia_QR_Code.jpg

This image is by Wikiwatcher1 who has posted it to Wikimedia Commons under a creative commons license - the original file details are here http://en.wikipedia.org/wiki/File:Wikipedia_QR_Code.jpg