Red shades tend to stand out more to the human eye than blue shades of the same saturation. As such a blended or "purple" map of the US, based on the popular vote will tend to look a little more red than blue in its hue, given the bias of the human eye. Therefore, I wanted to adapt the purple America map by Robert J. Vanderbei to test my hypothesis. Below are extractions of the blue channel and the red channel of the maps.

Note: a good example of this bias can be seen by looking at the larger version of the original red/blue/purple map and look in the lower left hand corner, at the scale. Pick the point at which you think it's a true purple. For me, it's between 2/3rd's and 3/4's of the way down from the top. Assuming that the proportions of blue decrease linearly going up and the proportions of red decrease linearly going down, if the human eye perceived blue and red equally and if pure purple were a 50/50 mix between red and blue, then the purple point should be halfway up the scale. It's not. This has two causes. First, it's because the actual 50/50 mix of blue and red is violet, not purple. The second is that the human visual system percieves red more strongly than blue, for an actual constant intesity. Evidence of this phenomenon can be found in any sensory and perception text or online at http://acept.la.asu.edu/PiN/rdg/color/color.shtml at the bottom of that page there's a response curve drawn for light of the same intensity and the percieved brightness, based on wavelength. If you compare the responses for blue vs. red wavelengths, though they're both relatively low, the red is stronger than the blue.

Therefore, while adding no new data, my splitting presents it in a way that makes the differences and proportions more clear to some observers.
 

This map is the blue channel -- the stronger a given county voted for Kerry, the lighter the shading will be on this map.	This map is the red channel -- the stronger a given county voted for Bush, the lighter the shading will be on this map.

Or, if you like it inverted...
This map is the inverted version of the blue channel -- the stronger a given county voted for Kerry, the darker the shading will be on this map.	This map is the inverted version of the red channel -- the stronger a given county voted for Bush, the darker the shading will be on this map.