Walk into any one of the 570 Orangetheory fitness studios worldwide, and you'll be struck by how, well, orange everything is.

From the studio lighting to the water bottles to the sneakers worn by much of the staff — the branding is pretty straightforward — but that is not the orange driving most sweat-seekers to work out at this fast-growing fitness company. The so-called orange zone is when you exercise at 84 to 91 percent of your maximum heart rate.