I found your question and this thread interesting so I did some research.
There are lots of warnings with good explanations of why propane tanks should be painted in a light
reflective color. It all comes down to heat absorption and the potential pressures that could be created.
There also appears to be legislation contained within the NFPA 52 on tank color. There are also warnings that each state can enact it's own laws that exceed the National legislation.
I am no expert but my interpretation to this is some places may not refill your tank if empty, also insurance may be void if there was a fire (we all know insurance looks for a reason not to pay)