The American Water Works Association recommends the following color scheme for painting hydrants to indicate their relative capacity.

The capacity is to be determined by flow measurements of individual hydrants taken at a period of ordinary demand. When initial pressures are over 40 psig at the hydrant under test, the rating is to be based on 20 psig residual pressure, observed at the nearest hydrant connected to the same main and when no water is being drawn. When initial pressures are less than 40 psig, residual pressures shall be at least half of the initial.

It is recommended that the hydrant barrel be painted chrome yellow, unless the water system prefers another color.