Does C stand for Colors?
The innumerable color combinations points that C might stand out as a color and in fact it might be partly correct.
The alternative theories
C = Confused product
Apple apparently were out of ideas to take on competition, so came up with 5C, a confused product; the sale figures will confirm or refute this theory.
C = Covert product
Apple knew from the beginning that 5C was a premium product, but still kept away from refuting the claims of cheaper iPhone. Make all to expect a cheaper version; show off at the end that it is a premium phone; covertly imply that Apple was and is a premium company which will never care for competition; increases brand value; increased sales - pretty cool covert idea.
C= Crafted by craftsmen
Apple's design manager speaks in the 5C video about converging of hardware and software as the prime factor in designing 5C. Some say it is a Jony Ive's pet product and the Apple's policy of harmonizing hardware and software has entered into design field with him.