Etymology
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de minimis 
Latin, literally "of little things," thus, "so minor as to not be worth regarding."
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red cent (n.)

type of copper penny, 1839, American English, from red (adj.1) + cent. Pure copper pennies were issued 1793–1857, then replaced by ones of copper-nickel and, after 1864, bronze. The old cents were disused, but the phrase remained colloquial as a mere emphatic of cent, usually in the negative (don't have a ... not worth a ...). "Red" has been the color of copper, brass, and gold since ancient times.

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black sheep (n.)
by 1822 in figurative sense of "member of some group guilty of offensive conduct and unlike the other members," supposedly because a real black sheep had wool that could not be dyed and thus was worth less. But one black sheep in a flock was considered good luck by shepherds in Sussex, Somerset, Kent, Derbyshire. First known publication of Baa Baa Black Sheep nursery rhyme is in "Tommy Thumb's Pretty Song Book" (c. 1744).
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