It's to do with grouping I believe. The actual colour isn't significant, it is just a visual aid to help you identify brushes that are in a group.
Try starting a new map, and add a couple of block brushes. Each will be a slightly different shade. Then select some of them and group them (CTRL-G). Deselect everything, and you'll see the ones you grouped are now all the same colour.
It's adjustable in Hammer. As grazr said, func_details are green, prop_statics are yellow, entities are purple, grouped brushes are white and so on. Normal brushes are some greeny random color.
If you shift-click and drag a brush to copy it, or if you clip a brush into two brushes, the new brush will keep the same color, allowing you to easily distinguish block types. I kept my walls with the default color, used a new color for the floors, etc. Works nicely.
I actually use the camera preview lighting sometimes. It's cool because it shows you the range of your lighting. It looks sorta darkish and your final lighting will look nothing like what you see but it's good at giving your your lighting range.
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