QR code on yellow

I wasnt sure if the QR code had to be black on white and aparently it doesnt, as long as there is a big enough contrast. The optics of QR codes are simple, and like any other bar code symbology. The more contrast the better. Contrast is usually expressed as a ratio, and that ratio varies. Anything below 20% may be unreadable, or intermittently readable (readable on some systems and not others), and 30% is a safer minimum.

Back in the day, people used a device called a verifier to scan codes to determine whether the bars and spaces were properly encoded, and whether there was enough contrast. These are available for QR codes, and any company printing QR codes in volume should use a verifier.