“Why does beer go through your system so fast? Becuase it doesn’t have to stop to change color!” is an old joke about the color of beer. It’s not known how old the joke is, but the following was cited in print in 1989:
 
Objoke:  Why does beer go through your system so much faster than water?
A:  Water has to stop to change color.
 
Sometimes a beer brand is specifically mentioned in the joke, such as Corona.
 
 
Uncyclopedia, The Content-Free Encyclopedia: Beer
“Why does beer go through you so fast?”
~ Sociologist Pee Wee Herman on beer. The answer is that it does not have to stop to change color.
 
Google Groups: comp.misc
Looking for Computer Folklore
Scott “The Pseudo-Hacker” Neugroschl
3/16/89
(...)
Objoke:  Why does beer go through your system so much faster than water?
A:      Water has to stop to change color
 
Google Groups: alt.support.divorce
Oh the joys!
Daisy
11/5/98
(...)
Do you know why beer goes through your system so fast?
Because it does not have to stop to change color.
 
Google Books
Country Music
And Other Stories
By Arelo Sederberg
Lincoln, NE: iUniverse
2007
Pg. 236:
“Gotta piss out the beer,” he said. “It goes through you so fast because it doesn’t stop to change color.”
 
Twitter
Steve
‏@behindallbars
Th reason #beer goes through your system so fast ... it doesn’t have to stop to change color     #bartending
7:19 AM - 24 Jun 2012
 
Google Groups: WANAKENYA
Re: [PK] ON A SERIOUS NOTE THOUGH
Kuria-Mwangi
12/28/12
(...)
Q: Why does Corona go through your system so fast? A: Because it does not have to stop to change color.
 
Twitter
Drunk Mellie
‏@Drunk_Mellie
Why does Corona go through your system so fast? — Because it does not have to stop to change color. http://ask.fm/a/a26db4ce
1:21 PM - 25 Nov 2013
 
Facebook
Mr. Crowley shared I fucking love science’s photo.
July 11 (2014) at 3:01pm ·
I’ve been told beer makes you piss quickly because it doesn’t stop to change color on the way down.