I've operated a rental B&W darkroom, and I had my own color darkroom;
though it WAS some years ago.

My feeling is rental color would be very hard to make profitable,
especially given the realities of todays digital marketplace.

First, it would be hard for amatuers to get good results. It takes a
while to get the "feel" of a color darkroom and get consistant results.
Most of the people who "just wanted to try it" would quickly get
discouraged; professionals would want their own facilities. Your
clientelle would be those in between.

Second, the startup and operating cost would be high. I think you'd
need an automatic paper processor like a Jobo; drums would be too
cumbersome. Not only are expensive, they're expensive to load with
chemistry - and you have to fill them even for one print.