Which can be difficult at times because they ask for proof of residency and don't accept black and white when that's the only proof I have access to.
okpay is rubbish at that. My proof of address had a small color logo on the top right and was black and white otherwise. okpay would reject it automatically... their color-detection algo is broken imho.
However, I loaded the pic in IrfanView, added a bit of color tint (red or blue? can't remember) in the color settings. It went through their automatic check and was soon after accepted.