I've always found that orange marks really well on coloured sheep and works well on white ewes which is handy in mixed bunches. Orange is also really handy because it seems to last much longer than most of the other colours. Shame my stock mark is red really for it's rubbish on black sheep!
P.s This Dougal isn't hebridean! Slightly better bred, carries more weight and lacks the 4 horns!