What might cuause a heater core to clog?

Seven days ago at just over 150K I had a leaking radiator replaced. When driving home that day the heater was working fine. Next day I drove about 110 miles without noticing anything wrong with the heater although the outisde temperature was in the 40s and I might not have noticed if the heat was not working. Car then sat for 2 days. Drove a return 110 miles on the fourth day. No heat (temperature guage at normal level). Returned to repair place. Thermostat in open position. Replaced thermostat. Then was told that heater core was clogged and needed replacing.

The discussion was then about whether or not the wrong color coolant had been added as this could cause the coolant to congeal and clog the heater core. (The cooland was changes about 13 months ago when the timing belt was changed. No coolant added since then.)

And thus this question. What besided adding the wrong coolant (the only time that might have happened was when the coolant was topped off after replacing the radiator) might have caused the heater core to clog?

You can see why I'm wondering re the adding of the wrong coolant. Nothing seems wrong this fall with the heater. The problem arrived soon after that new radiator was installed. So I'm in search of a third party opinion.