Are you low on coolant in your radiator?
Change you oil like stated above if you get same color then you have a leak somewhere, I would assume it to be from the seal on your water pump shaft. If not then it could be very old oil from the previous owner. I used to have a YZ 80 that i beat on pretty had and would drown it going through deep mud puddles in the woods. This was back before i did regular maintenance but use to ride in these trails every week and decided to change oil one day after and it came out almost white like milk. Turns out it was do to water getting into the case from the overflow tubes.
Anyways i would start with oil change, top off coolant, and go from there.