I cannot pretend to be impartial about the colors. I rejoice with the brilliant ones, and am genuinely sorry for the poor browns.
What is the pinnacle, the sunnum bonum, or (don't speak Latin?) the ultimate achievement of compositing? Pulling the perfect matte? Creating a convincing effect seemingly from nothing? Leaving the last doughnut at dailies for the effects supervisor? Those are all significant, but they pale in comparison with the ability to authoritatively and conclusively adjust and match the color of a foreground to a background.
Without this skill, you will not have ...