its either coolant or water. or even just old oil. usually water in the oil makes a greyish color, where old nasty oil is light brown like your describing. however a tiny bit of water could have gotten into the transmission via the crankcase breather tube, have you gone through any deep puddles? also, it is possible to force water in with a pressure washer. highly unlikely but possible ;)
check your radiator fluid, if it is low that's your problem. *brain fart* i cant remember which gasket/seal it is that would cause coolant to get in.... hmmmm.... :think:
if it were me, i would change the oil and see what it does. i change the tranny oil every 3 (significant) rides, or about every 6-8 hours ride time and its always at least a little contaminated, every time you stab the clutch those plates are wearing out and the slop goes straight to the oil!
Rob