Colors will never work, or legally be accepted.  Too many people are colorblind, and you can't normalize something if you have two definitions (location AND color) of one significant digit.  In case of conflict, which one has priority - the color or the location of the digit?

You do have the proper breakdown though.  2 (space) 3 (space) 3

9.87 654 321

It wouldn't be hard to implement spaces into any GUI, and it easily identifies how significant (or insignificant) the numbers are.  (Notice I stayed away from any naming references.  )