When you edit, and especially when you colorize, you want to do it in a relatively dim environment. And if you cannot achieve that, you at least don’t want any direct light falling on your monitor. That can really screw up the color your eyes perceive. If you are using a calibration tool like Datacolor’s Spyder (reviewed here) it will notify you when there is too much light hitting your monitor.
But even if you don’t have a tool for that, it is always better to work with a hooded monitor. Hoods for 21 inch screens run for about $30–$35, which is fairly reasonable, but I could not get one locally and did not want to wait. So here is how I made my $5 hood in 5 minutes flat.