Pied Mastiffs occur rarely. Other non-standard colours include black, blue brindle, and chocolate (brown) mask. To allow for the rare exceptions we must include 'b' (brown mask and possible brown brindling), 'd' (blue mask and possible blue brindling), 'sp' (pied spotting), and perhaps 'a' (recessive black). The possible combination of homozygous brown and homozygous blue is a pale brown referred to as isabella in breeds where it is relatively common.