Ian, the red, white and blue of Mercury is evident on all the planets when they are low on the horizon.
It's due to atmospheric refraction.. because you are viewing through more of the atmosphere, and (I think) the different wavelengths of light might get bent in slightly different directions. It's an imagers nightmare and that's why it's always best to image the planets when they are directly overhead, where there is less atmosphere to image through and no atmospheric refraction to speak of (not to mention, seeing is usually best overhead too).
Have a look at this Mercury image I took from last week, you can see the individual raw frame has the red/white/blue that you mention. Luckily, registax has an RGB shift function which re-aligns the Red and Blue channels to make them match up, and produced the much better stacked image.
I'm sure someone will (please) correct me on anything i've said that's incorrect.
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