The temperature settings are in Kelvin. Temp settings that should be familiar are 6500 (daylight) and 2800 (tungsten - incandescent).
9300 tv screen
This is a good way to imitate photographic white balance differences, but most likely, you'll just want to make pretty images with white balance set to neutral or daylight. So another more interesting example, is to use the portal light in conjunction with the physical sun/sky system. You don't have to plug it directly into the portal. This will enable higher FG accuracy with fewer rays. Below is an example.
The portal light allows the entire environment both direct sun and indirect sky, to act as a soft direct light. Along with that, comes accurate area shadows as well as improved FG. The mental ray architecural docs have some great info about this and should be ingested if you want to take this further.