The people on this forum really amaze me sometimes.
The poor guy asks between an M5 and M6 and you guys turn the topic into one about audis and porsches and astons... it's absolutely ridiculous.
I recently posted a topic asking what car I should lease within a given price range, and what do I get? Comments telling me to buy, not lease, and the thread ends up becoming an argument about whether leasing or buying is more economical and toyotas and hondas got thrown in the mix.
Stick. With. The. Topic.
BMW Diet - my advice is simple. Drive both and see which one you're most comfortable with.
The M6 looks more exclusive and the M5 is more of an underdog. If you need space, no question go with the M5. But if not, simply drive both and see which one puts the biggest smile across your face.
You asked about color, and this is completely subjective, but I really love the M5 in black, I don't think any other color suits the car better.
Hope this helps.