Not so simple one, color of fire hydrants mid 20th century. - Model Railroader Magazine

Interestingly enough, for modern era US modelers, there is indeed something of a standard color code for fire hydrants: NFPA 291, which has been around over a decade

In an effort to make it easier for firefighters to know what a specific hydrant will supply, the National Fire Protection Agency (NFPA) recommends that fire departments and water districts follow a set standard of color-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.

Also, as mentioned, public water supplied hydrants should be yellow, private water supplied (from local wells) hydrants should be a dfferent color (red is recommended), non-potable water supplied hydrants should be violet, and inoperable hydrants should be...black (which would be an issue with New York City fire hydrants, most of which are black w/ silver caps as mentioned in a previous post).

Philadelphia fire hydrants are often a orange/red color with a green cap (I used orange and a pale green on the model hydrants for my module), although I did see some all orange hydrants - so this would mean (IF the Philly FD is following the NFPA standards), the orange green hydrants have good water flow, and the all orange ones have so-so water flow. Red caps were denoted as "poor" water flow in a different list, but since red can often fade to an off-orange in the exposed outdoors where fire hydrants reside, perhaps they should have chosen silver or grey for the poor water flow color).