Coolant discolored and plastic resevour tank is darker

oil cooler can cause it also.

 

I change my extended life coolant at 300k. Sent coolant in for analysis. Even though the coolant says good for 600k
Found mine was lacking in protection and coolant flush was recommended.
Mine was also dark. I never go to recommended intervals anyway.
May try sending a sample to a lab. My Local Cat dealer has a full service lab. They do testing for a variety of fluids

 

Found one of our 388's had a transmission cooler above the bell housing. It was bad resulting in trans fluid in coolant and coolant in trans. Replaced it, flushed and refilled both. Replaced trans within 6 months.

 

I had a tranny cooler go and mine didn't last 3 weeks after it filled with coolant that trans cooler got expensive really quick.

 

Maybe a leaking egr cooler.

 

I agree get a sample and get it sent into the lab for analysis.

I would also change it out immediately with new coolant and a filter.