In the U.S. state of Hawaii, there is a belief that state law prohibits the use of standard road signs on private property. Whether this belief is true or not is a matter for a Hawaiian lawyer to advise on. In any case, blue STOP signs are attested. Many other road signs that ordinarily feature the color red could also have a blue counterpart, for example:
|R5-1||R5-1a||R3-2||R7-1||with reflective strip|
Along the same lines, green might be changed to brown, and blue to red:
These images are based on those in the PDF version of the 2012 revision of the 2009 edition of the Manual on Uniform Traffic Control Devices produced by the Federal Highway Administration of the United States Department of Transportation. The legend below each sign is its MUTCD identifier.
Colors were changed using GIMP, which made the process quick and easy.