Old English grighund (West Saxon), greghund (Anglian) "greyhound," probably from grig- "bitch," a word of unknown etymology, + hund "dog" (see hound (n.)).
The first element in the name apparently has nothing to do with color, as most of the hounds are not gray, but the exact sense of it must have been early forgotten, as it has been long associated with the color in popular imagination. In some Middle English forms it appears to be conformed to Grew, an old word for "Greek" (from Old French Griu). The Old Norse form of the word is preserved in Hjalti's couplet that almost sparked war between pagans and Christians in early Iceland:
Vilkat goð geyja
grey þykkjumk Freyja
[translation "I will not blaspheme the gods,
but I think Freyja is a bitch"]
"purse-like tegumentary investment of the testes and part of the spermatic cord; the cod" [Century Dictionary], 1590s, from Latin scrotum, probably transposed from scortum "a skin, hide" (see corium), perhaps by influence of scrautum "leather quiver for arrows."
"Isn't the sea what Algy calls it: a grey sweet mother? The snotgreen sea. The scrotum-tightening sea. Epi oinopa ponton." [Joyce, "Ulysses"]
1874, also polio-myelitis, coined by German physician Adolph Kussmaul (1822-1902) from Greek polios "grey" (from PIE root *pel- (1) "pale") + myelos "marrow" (a word of unknown origin) + -itis "inflammation." So called because the gray matter in the spinal cord is inflamed, which causes paralysis. The earlier name was infantile paralysis (1843).
In many respects, also, this affection resembles the acute spinal paralysis of infancy, which, from the researches of Charcot, Joffroy, and others, have been shown pathologically to be an acute myelitis of the anterior cornua. Hence, for these forms of paralysis, Professor Kussmaul suggests the name of 'poliomyelitis anterior.' [London Medical Record, Dec. 9, 1874]
Polioencephalitis (also poliencephalitis) "inflammation of the gray matter of the brain" is by 1885.
Old English names for the creature were the Celtic borrowing brock; also græg (Middle English grei, grey). In American English, the nickname of inhabitants or natives of Wisconsin (1833).
"a vigil beside a dying person," 1865, from death + watch (n.) "a watching." The death-watch beetle (1660s) inhabits houses, makes a ticking noise like a pocket-watch, and was superstitiously supposed to portend death.
FEW ears have escaped the noise of the death-watch, that is, the little clicking sound heard often in many rooms, somewhat resembling that of a watch; and this is conceived to be of an evil omen or prediction of some person's death: wherein notwithstanding there is nothing of rational presage or just cause of terror unto melancholy and meticulous heads. For this noise is made by a little sheathwinged grey insect, found often in wainscot benches and wood-work in the summer. [Browne, "Vulgar Errors"]
"brownish-purple," literally "flea-color," 1787, from French puce "flea-color; flea," from Latin pucilem (nominative pulex) "flea," from PIE *plou- "flea" (source also of Sanskrit plusih, Greek psylla, Old Church Slavonic blucha, Lithuanian blusa, Armenian lu "flea").
[T]he couleur de Puce, or flea colour, and the couleur de Noix, or nut colour, are the reigning winter taste. [Westminster Magazine, January 1777]
Perhaps so called as the color of the scab or stain that marked a flea-bite; flea-bitten was a color word in English to describe whiter or gray spotted over with dark-reddish spots (by 1620s, often of the skins of horses, dogs, etc.). That it could be generally recognized as a color seems a testimony to our ancestors' intimacy with vermin.
OED sees no connection between this word and obsolete puke (16c.-18c.; hence Shakespeare's puke-stocking) as the name of a dark color of now-uncertain shade (Century Dictionary says perhaps reddish-brown, OED says bluish-black or inky; others suggest grey).