Whist Variant with points suit
After some play-testing with friends, the complexity was intimidating, and so we settled on these rules:


  The last card is dealt face up, and it is the point suit.  
  
  There is no trump suit.
  
  You cannot lead with a point suit until a point suited card has been discarded.
  
  When a point suit is led, any other card can trump it. 


The results were interesting: Once players caught on to the game, they would discard points often rather than hording them, hoping their partners can pick them back up.  Teamwork seemed more vital in this variation than any trick taking game I've played.

It turned out that having a both trump suit AND an anti-trump suit was an unnecessary complication. The goal was to acquire point-cards which could be trumped by any card, and so having a trump suit felt superfluous. 

Alternatively, we played the following variation:


  The trump suit is the last card dealt, the point suit is the other
  suit of the same color as the trump suit. 
  
  The point suit cannot be led until a point suited card has been
  discarded.
  
  The point suit is a normal suit, and is not trumped by  any suit
  except the Trump suit.


This variation worked well, and it satisfied my requirement that the game make use of the fact that there are four suits but only two colors.

However the first variation in this answer seemed more fun.  I'm not sure why.