Is the coolant dropping at all? Are you noticing rapid changes in running temp of the engine? Is there anything like oily deposits floating in the coolant of the header tank?
If no then don't worry its just many years of general crud build up which is normal, more so if the coolant hasn't been flushed to service intervals and the correct type of coolant used. With the car being at least 16 years old now that coolant looks quite clean still.