After posting my previous comments, I did some checking. (I googled 'painting propane tanks')
Apparently, there's nothing preventing you from painting your tank. There are, however, some specific guidelines.
-- Use light colored or reflective paint. Dark paint will absorb sunlight, possibly raising tank temperature to dangerous levels.
-- Info stamped into valve guard or base rings must remain visible and readable.
-- Remove rust and loose paint manually.
-- Do not use harsh chemicals or abrasives that could compromise tank integrity. Do not use rust converters, rust removers, sandblasting, or aggressive sanding. (In addition to other difficulties, I would not use one of my electrolytic rust and paint removal tanks.)
-- Clean tank with soap and water, rinse with clean water, and allow to dry thoroughly before painting. (As always, I recommend dish soap ... water should be, at most, lukewarm.)
-- Prime before painting unless you use self-priming paint.
-- Do not conceal rust pitting or anything that indicates possible tank integrity compromise.
There appears to be paints specifically marketed for painting propane tanks. I didn't research further to determine if these are indeed special paints or this is merely a marketing tactic.
Removal of surface rust only is acceptable. Any signs of rust pitting (or other damage) means you should remove the tank from service due to safety concerns.
