Being the friend who thought of this, I thought of a few additional reason why such as system is a good idea. I think that colours may be useful because they will both make the site more colourful and also increase the memetic nature of the site which may help build a culture.
I don't think Hubski needs to be more colourful. The Grey theme is complimented well by the simple touches of orange and blue. If the hubski wheels were to be given stages of colour depending on the amount of shares, then the colour set would have to be monochromatic. Otherwise any page would look very messy if there were wheels of every colour scattered across the page. I really like the idea of colour stages but I believe the colours would have to begin with a very light blue, or even a light grey, then progress into slightly darker blues as it was continued to be voted on, a post which may have hundreds or even thousands of shares eventually may even reach the colour black. I really like the idea, but my point is that the wheels would have to be monochromatic. Making Hubski more colourful isn't necessary.