The Umbrous was discovered in the USA in 1998 and is a darkening gene. It does not exist on its own but combined with any colour it produces a "sooty" version of the colour. The effect of the Umbrous mutation on the normal colouring is to turn the belly from ivory to a sooty grey/brown colour and the normal brownish-grey back and sides become darker.

The Umbrous can be combined with any colour to produce a "sooty" variation.