Q: Radiator seems to be clogging: it started just 2 weeks after flushing it!

Hi. As mentioned above, i flushed the radiator 2 weeks ago, added antifreeze and distilled water (50/50), changed the fan clutch and pressure tested the system. Everything seemed to be perfect until i checked the mixture 2 weeks later.. Note that the coolant level is still amazingly at the right mark(no coolant loss). However, a light brownish color started to appear day by day. The light blue color changed into light brown which i suppose means that the radiator is starting to clog. (a slight overheat incident took place while climbing a hill which i assume is the effect of the coolant color.) My question is .Are the 30 year old car (corroded) engine coolant paths responsible for the brown color? if yes, what can be done to solve it...thank you!

My car has 154000 miles.
My car has an automatic transmission.