Just to clarify, 'Black and Tan' is usually a cover-all term for both 'Temporary Constables' and the later 'Auxiliaries'.
Due to a shortage of police uniforms, the TC's were generally supplied with a mixture of army khaki and bottle green (bordering on black) RIC uniform, when uniform supplies normalised, the full dark green uniforms were issued but the B 'n T nickname stuck and was used to refer to both forces.