What do the numbers on the collars of Japanese military uniforms mean during World War II?

During World War II, the numbers on the collars of Japanese military uniforms were the serial numbers of infantry regiments. For example, the Arabic numeral 3 refers to the Third Infantry Regiment.

The colors of the four-five collar are: infantry red, artillery yellow, cavalry grass green, engineering brown, trench dark blue, aviation sky blue, military police black, military band blue, staff officer white, management department tea silver, military medical department dark green, veterinary department purple, legal department white and technical department yellow.