I've searched through my saved tips 'n' tricks file, and can't find anything on the subject. I seem to recall that the method I saw was Photoshop specific, and since I don't use PS that would explain why I didn't preserve it.

A .BUM map has information only in the red and green channels: the blue channel is always zero, which explains the khaki hue. From memory, there's a small shift of a few pixels between the two channels, which you'd have to undo to perform an accurate deconversion. If you have an old asset which includes both .JPG and .BUM maps you may be able to reverse engineer the process.

However, bear in mind that there was a bug in the BUM conversion in the Windows versions of Posers 3 & 4. My less-than-scholarly analysis is preserved here, but bear in mind that the links in that article have rotted since Renderosity changed their forum links.

Alternatively, you could just convert those remaining BUMs to greyscale and have done with it. The whole thing seems to be more trouble than it's worth. :)