Not black in my mind, though yellow in my legs. "To wear yellow hose" meant "to be jealous"; Malvolio apparently means that even though he is wearing yellow hose, he's not jealous, which would imply that he's sure that Olivia really loves him. And he wants to draw attention to the fact that he's wearing the yellow stockings that Olivia likes to see on him, as he has been deluded into believing. .