We Have Watched Many Films Based In America,

and one of the many things that we notice is fire hydrants,

 normally painted yellow, 

 but some times red,

 and often with different coloured caps,

but it appears that hydrants can be painted in numerous colour combinations and here is why, due to the numerous different supply systems throughout the United States, there can be a wide range of the available supply of water pressure, from 500 GPM or less to over 2500 GPM. the National Fire Protection Agency (NFPA) recommends that fire departments and water districts follow a set standard of colour-coding, known as NFPA 291, 

it says fire hydrants using public water supply systems should be painted chrome yellow, and their tops and caps should indicate the available GPM. Below 500GPM should be red, 500-999 GPM should be orange, 1000-1499 GPM should be green, and 1500 GPM or more should be blue, NFPA standards also require that private hydrants placed on or near public streets be painted a color that will distinguish them from public hydrants, furthermore OSHA standards require sources of non-potable water be painted violet.

 but of course street artists can give the hydrants a make over, but at least we now know why in the USA fire hydrants are different colours.