Oil in coolant reservoir on 1998 Jeep Cherokee

I had the tranny cooling radiator replaced. I was getting light-brown sludge in the rediator and reservoir. Now I have dark-brown sludge only in the reservoir; radiator seems clean ( unless the sludge settles to the bottom). What's up?

Asked by for the 1998 Jeep Cherokee
Clean out the coolant reservoir, maybe the oil from the trans cooler settled in the reservoir.