Seems they tried to walk the line between dice specifically for Arkham Horror and general use dice, and ended up with something that's not going to entirely please anybody. Making the 5 and 6 a different colour might have been a more effective solution; those elder signs do add an awful lot of clutter.
Think I'll just stick with my plain old squamous green dice. 12mm dice are really the only way to go anyhow, given the bucketloads you occasionally have to roll. Standard size dice tend to leave a path of destruction across Arkham worthy of Shudde-Mell.