Cool – but illegal. The whole point of fire hydrants being red (typically) is so that firefighters can spot them quickly during an emergency.
They are all sorts of colors here (many of them green – and blend in with the grass >_>) But I would think an R2 decorated one would definitely cause it to stand out.
They do come in a variety of colors: red, green, yellow, etc. They are color coded to tell firefighters the pounds of water pressure they can expect out of each particular hydrant, some produce more than others.
Knowing you're at a low pressure hydrant, and may need to capture another to fight a fire, is kind of important, which is why painting them is illegal.