"come_mon_come_mon!" wrote...
I'm compiling a long list of percentage data stored in a single column
of excel worksheet (say it in column A). I want to label different
ranges of percentages by different colors e.g. 0-20% by yellow, 20-40%
by orange, 41% above by red, etc. How could I do that with excel
function ?

You can't do it directly with functions, but you could use Conditional
Formatting (menu command: Format Conditional Formatting...).
However, conditional formatting is limited to 3 conditions plus the
default cell formatting, so only 4 different states.