if it's yellow, let it mellow; if it's brown, flush it down

[Robyn] Griggs [Lawrence] tells them [the audience] they can make a difference without spending thousands of dollars: Line-dry clothes, vacuum refrigerator coils, and reuse greywater—water that’s been used once but is clean enough to use again—to irrigate the lawn or fill up the toilet tank. She recites her own family’s mantra regarding toilet-flushing: “If it’s yellow, let it mellow; if it’s brown, flush it down.”

2002 October 7, Liz Stevens, “Green day”, in Fort Worth Star-Telegram, 97th year, number 165, Fort Worth, Tex., →ISSN, →OCLC, page 11D, column 2

The motto for our bathroom is “If it’s yellow, let it mellow; if it’s brown, flush it down.” Yeah, that is the bathroom—a small commode that flushes with help of a bucket of water.

2004 July 22, Sarah Elizabeth Baker, “Their needs are so great—it breaks your heart”, in The Commercial Appeal, 165th year, Memphis, Tenn., →ISSN, →OCLC, page EM7, column 1

Never mind 'If it's yellow, let it mellow – if it's brown, flush it down.' My motto is: 'If it's brown, spread it on the ground – if it's yellow, recycle it like a good fellow.'

2008, Ethan Greenhart, Can I Recycle My Granny?: And Other Eco-dilemmas, →ISBN

Here is what I will say about the “If it's yellow, let it mellow; if it's brown, flush it down” rule: this is fine, providing you drink a lot of water.

[2014, Vanessa Farquharson, Sleeping Naked Is Green, →ISBN

He [Jimmy Goodson] also passed along some water-saving tips to customers from residents Ingrid and Doug Scott. “If it’s yellow, let it mellow; if it’s brown, flush it down. Urine is a sterile liquid and the toilet is not there for the purpose of Fido getting a drink or Junior floating his rubber ducky,” the couple said.

2015 September 3, Dawn Gagnon, “Castine suffers water shortage: Town buying from Bucksport”, in Bangor Daily News, state edition, Bangor, Me.: Bangor Publishing Co., →ISSN, →OCLC, page B2, column 2