Learn English – Why do we “paint the town red”

The book American Slang gives the following entry for pan:

1. The face. MUG. Too great for them to keep their pans shut. (1923+)

This seems to be when pan first entered American slang. From there, it went on to form words like deadpan (recorded in 1927). Only one site gave a hint to the actual origins, though. In this question about the origins of "deadpan", it is written that:

The key to "deadpan" is the use of "pan" as theatrical slang for "the face" (reflecting the use of "pan" to mean "skull," found as early as 1330).

Going off of this, there seems to be some support. The Word Origins page for skull says:

The Old English word for ‘skull’ was hēafodpanne, literally ‘head-pan’. It has never been firmly established where its Middle English replacement skull came from, but is seems more than likely that it was borrowed from a Scandinavian language (Swedish and Norwegian have skalle ‘skull’).

So the old word for skull gave pan, which was adopted to mean face. From there, pan entered American slang in the early 1920s.

The two meanings included in the Oxford English Dictionary for eyewash are:

1. A wash or lotion for the eye.

2. colloq. Something that is intended to obscure or conceal actual facts or motives; humbug, blarney; nonsense; something said or done merely for appearance or effect; spec. in Mil. slang = bull n.4 4.

The colloquial sense has written use dating from the 1800s:

1884 C. T. Buckland Sketch Social Life India ii. 45 Most officers of any tact understand the meaning of eye~wash.

The Wordsworth Dictionary of Phrase gives some explanation of why eyewash also means to "obscure something", writing:

So this "fulsome adulation" is figuratively putting a wash over someone's eyes. There are no clear references on where this "wash" came from. It could be from the first meaning of eyewash--if you are flattering someone, you are figuratively putting some type of wash in their eyes that prevents them from seeing your true intentions. It could also be from a meaning of "wash" as in a coat of paint (a wash of watercolor), so you are again obscuring someone's vision. I would guess that the slang sense was actually a figurative application of the first sense, however.