It would be just fuel as the coolant heats the diesel fuel. Remember in a warped head situation a clean up cut allowed is not very much. A thicker head gasket may be available though.

Still you have to know the limits for even that. I am not suggesting you not buy the car but just be prepared.

The miss I would like to isolate before head removal for example. I do not suspect the engine got really seriously overheated as that usually takes the temper out of the rings and you have no problems related to that.

Since you do not I suspect a more moderate overheat. I always use to ask the seller if it blew all the coolant out for example.