I admittedly have an addiction to workout capris. I buy them in a ton of colors and patterns. But I also hit the gym 3 days a week and run 3 other days, so I actually do need multiple pairs of workout pants unless I want to be a slave to the laundry.  But even if all I had were plain black yoga pants, I'd still work out. (In fact, that's what I wore when I first started. Plain black Target yoga pants.)