Hey,
I think there are few ways to do this, but I would do it like this :
First insert a text field to report where you want to show the colors. Then right click the text field and select format text.
Go to Border tab, and there press the x-2 in backgroud.
Put something like this to the formula field :
if {table.fieldname} =1 then green else
if {table.fieldname} =2 then yellow else
{table.fieldname} =3 then red
- Jyri