Some set bonuses improve as you improve (green, blue, purple, yellow) each item of the set.  But many do not.
The Song of Lamae set is an excellent example of a set that responds very well to item improvement.
A 5 piece level 50 set of Lamae, with all five pieces plain white, gives set bonuses: "Adds 15 Health Recovery.  Blah blah health less than 30% deals 212 damage to attacker and heals you for 212".
A 5 piece level 50 set of Lamae, with all five pieces YELLOW, gives set bonuses: "Adds 19 Health Recovery.  Blah blah health less than 30% deals 260 damage to attacker and heals you for 260".
A good example of a set that really should change as you improve the set pieces BUT DOES NOT is Whitestrake's Retribution.
A 3 piece level 50 set of Whitestrake's, with all three pieces plain white,  grants a 189 damage shield.
A 3 piece level 50 set of Whitestrake's, with all three pieces GLORIOUS YELLOW,  grants a 189 damage shield.
Why won't the damage shield value inch up to something like 200 and make me happy?  Is it a bug?  Is it by design?
There's many other player made sets with bonuses that are completely static (like Magnus' or Seducer's).  And then there are partially static sets like Hunding's Rage; the 3 piece bonus improves while the 5 piece mega bonus does not.               
                Edited by BirdUp on May 16, 2014 12:58AM