As you said it is only to make them **stand out**, you mostly remember what your skill does but there are many occasion you want to recheck some of the values, one of them often being skill damage comparison.

 - Some of the values change by using **runestones**,but note that some
   skills completely change the way they work,so adding runestones
   actually changes the text along with the numbers.

 - Some values are **constant** during the whole game like some
   cooldowns,stun duration,mana costs,number of hits...

 - In D3 many skills use weapon damage **percentage** instead of actual
   skill damage to balance the game between melee and magic classes as
   they gather better and better equipment and progress levels.

Because of the use of percentage on most skills there is not so many variables and that is why numbers are green for "cosmetic" purpose instead of indicating the actual variable.

-There are rumors that some values scale with the player level,I didn't seen any myself,and I don't see a need for them to scale based on D3 skill system.