Prevent grep from removing the color

[Yang Zhang]

Hi, I'm interested in cascading a bunch of coloring greps to 
syntax-highlight lines.  E.g. given the string:

  user3 changed from Online to Offline

I'd like to be able to issue something like:

  GREP_COLORS='...white...' grep user3  |
  GREP_COLORS='...green...' grep Online |
  GREP_COLORS='...red.....' grep Offline

so that I see (in my made-up markup language):

  <white>user3</white> changed from <green>Online</green> to 
<red>Offline</red>

However, it appears that grep always resets the coloring on the 
non-matching portions of the line.  Is there any way around this?  Is 
there a more suitable tool (perhaps some general-purpose syntax 
highlighting tool)?  Thanks in advance for any help!
--
Yang Zhang
http://www.mit.edu/~y_z/