Header/expansion tank overflowing with black coolan..

Hi there I've got a niggling little problem with my Laguna.. Just gone out to it to day to top washer bottle up ( as the misses had drained it) I've noticed that the coolant in the header/expansion tank has gone from blue colour to black plus with the engine running the header/expansion was slowly filling up to the point of overflowing.

Would someone shed some light on this as I'm think it could be one of few things ie radiator,thermostat,water pump ect ect.