[Jprogramming] Colors in grid columns

[Donald Pittenger]

As best I see it, the CELLCOLORS assignment variable in grids seems  
row-oriented.  But...

Here's what I'd like to do:

Have a 3-column multi-row grid that initially has all backgrounds in  
one neutral color.  If the user clicks on a cell in the first column,  
the cell turns red.  In the second, column it would be orange or  
amber and in the right-hand column green -- a stoplight arrangement  
related to an application.

How can CELLCOLORS be structured to yield such an arrangement?

Also, where and how might CELLCOLOR related to this?  Is it needed?   
Is the indexing Boolean or otherwise?

This is not clear (to me, at least) from either the Wiki or the lab.

Thank you,

Donald Pittenger