This might help: provided that you have some block geometry like a hollow box surrounding your level, make sure there is at leasr one static light with its range intersecting the block geometry somewhere in the level. The light can be dark and somewhere out of sight, but its colors have to be set above 32.

It has to do with the default setting used by the engine at startup, and was discussed in another thread. I'll try to find it.

cheers,
tindust