Kink.com had a sub genre called “everythingbutt.com” that was mostly enema and anal play based. I was lucky enough to be good friends with the head of everything butt, Lochai. The first day I met him, he offered me a job at kink.com, but that is a different story for another time.
Lochai was discussing enema play on film with me at great length, and he was telling me that enema solutions on camera were tricky because there are only so many colors that show up on camera, but aren’t also off putting for the viewer.
Right off the bat, green, red, yellow, and brown were ruled out. Red looks like blood, which is not sexy to see coming out a bottom. Yellow looked like urine, and green was also swampy and displeasing on the way out. Brown was obvious- they didn’t need to add color to get that result.
This left them with a hand full of color options for enema solutions like clear water, soapy white, milk, and blue dye. For expulsions, those are all acceptable colors to see on camera, but obviously blue and milk shows up better on camera for expulsion scenes. Thus, why you see milk used so often in enema movies.