To put it bluntly, unless you're some kind of level editing god, have the standard HSL spectrum memorized and can convert those values on the fly, choosing the light color you want in UnrealEd can be a pain in the ass.
Ulight to ease your editing burden! (somewhat)
Ulight is a small FREEWARE application written in Visual Basic 6 that allows you to open the Windows Color Common Dialog
Box and pick the color you want. It then converts the color values to the approximate Hue, Saturation and Brightness values you enter in UnrealEd to acheive that light color.