the colours i guess? one way to better match would be to bounce the flash, which would usually even out a bit of the colours. but sometimes, the ambient lighting is so strong that the colour cast would be there.
in a orangey warm setting at night OUTDOORS, say with a warm building as background (say 50-100m away), you shoot subject with flash. so subject becomes ghostly white (ethereal looking) compared to VERY warm background.
another case is when background is lighted by orangey street lamps.
adjusting white balance does not work because your subject is cool, and background is warm.
seems like a flash filter is the only way, have not tried it out though. anyone has better knowledge or ideas?