Determine Dominant Color of Picture

I am trying to come up with an algorithm to determine the dominant color in an image (either taken from a devices camera or by selecting an existing photo in the photo library). I have written an iOS 8 application in Swift that can grab the RGB value of each pixel in the image, but I don't really know what to do from there.

For pixels that have a distinct dominant color, say RGB(230, 15, 30), it's pretty easy to determine the dominant color. However, I don't really know what to do for pixels that have RGB values where 2 of the 3 values are similar, say RGB(200, 215, 30).

My original thought was to keep 3 counters (one for each color) and add each pixels corresponding RGB values to that counter. At the end I would divide each counter by the total number of pixels and that would be the dominant color. However, like I mentioned before, when the results are close to each other I can't say that one color necessarily dominates the other.

Just looking for some thoughts and suggestions