Luckily I don't care about what colour the phone is and actually really like the white version of it now, but I can see people being annoyed if they are changing deals around all the time.
What it will be is some manager of the sales team seeing what is selling more that day. if more black phones are being sold they may have a surplus off white phones building up and therefor the best thing to do is give them an offer they can't refuse to get the white ones shifted. then the next day they'll swap the offer to black phones. maybe.... I might just be talking rubbish too!