We may never know what the "C" actually stands for. There are any number of words that fit the description of the newest member of the iPhone line — the most obvious being "colorful," as used in its tagline — but Apple has been somewhat coy about its true meaning.
I have my own theory. I don't think the "C" stands for any of the words that have been bandied about. I think it's more abstract than an simple adjective, a word or words that speak to the iPhone 5c's importance and what it represents to Apple, something far more personal to its namers than appearance or price.