I'm reading through this, and I don't think the symbolism of colors is really the point here. I'm sure this (like so many biblical phrases) can likely be traced to some kind of Hebrew idiom, which I could look into..
But I don't think it's strictly necessary to get the message. Whatever it meant for a person to swear by their head, we're not supposed to do that. We CAN understand that we are unable by trying to make a single hair on our heads go (naturally, from the root) lighter or darker.. it simply it what it is.
The more important theme of this passage (Mat 5:33-37) is simply that we are not to take oaths. I was relieved to find, for example, that my State has allowed "affirm" to be used in place of "swear" when a person takes the stand. I was quite worried about this, because I don't belive in taking oaths, due to this passage.
But the message is more than just refraining from the activity of swearing. We are being charged with the responsibility to live up to what we say, so that our "Yes" really DOES mean "yes" not "maybe" or "if I have time." Personally, I think that's a much richer message anyway, that can lead to years of personal growth, if we allow it shape our lives.