I can't find anything in the game about this, and it's probably nothing but...
I notice when I log in, my Lock (only level 36) has three Soul Shards, and they're the same colour. (I have a habit of casting [Soul Harvest] before I log out, it's always nice to log back in at full health and mana and shards, etc...).
But after I've used a few Soul Burns, and then replenished my Soul Shards, I notice that they are sometimes a different colour. Or one of the three might be a different colour (either lighter than the other two, or darker). Is there any significance to this?