I would expect colors are a function of when it was made, I don't know.  One batch of heat sinks were one color, another was another.  When you plug it into cgminer you'll see a serial number, but I'm not sure those were actively implemented then.  My 2Pac is 10010660, the heat sink is silver (natural aluminum), the circuit board is green.  My NewPac is the same colors.  Maybe there was a certain batch for a specific reason.  Later is probably better because improvements come along generally.