Because Digitec can make more profit. Supply demand has nothing to do with colour or function (see mobile phone colours).
But if too much is still too little, the prices will be increased. I bought the black one, it's easy to clean because it's not as susceptible to dirt. The silver one and you're polishing.
I also got the Sage Smart Grinder Pro mill in black. Have a look at my price comment on this.
That's why I also buy at M-electronics, which is also part of Migros = identical prices, but I still have Cumulus points with a 2x voucher etc.