horizontal line in table row - how to color it?

[Wolfgang Engelmann]

Hi,

how can I color a horizontal line in the row of a table (inserted with 
insert>formatting>horizontal line)

I need it to indicate the procedure in an experiment:

Top row shows various times of the day, row below shows e.g. a treatment 
with red light from 6 to 12 o'clock (should be red), far red light from 
12 to 18 (should be violet).

Wolfgang