it is like using colors for mark every reserve item. For example, we mark your coins as 'yellow' in our database. Next time when you use our service, we send you coins from range of others colors, like 'brown', 'red', 'white'... but not from 'yellow' to be sure that you do not receive part of your previous coins.
Ok, but does that not mean that tracing is a hell of a lot easier? I mean if you know that person x has the color yellow, then you can just search after yellow and will find all the bitcoins he has sent in? Or is it something that i have misunderstood maybe?