Getting ready to install new aluminum radiator.  Yellow premium coolant is less than a year old, and though already filtered thru a plastic coffee screen, it is still a very dark rusty-brown.  Hate to dump it (quite expensive) since the old block (and retained coolant) will likely just discolor any new coolant also.  But, if the micro-rust is apt to eventually adhere & accumulate in the radiator passages (and restrict flow), might possibly be worth the expense to minimize risk of any future negative consequences (?).  Opinions?
P.S.  BTW:, also installing an inline hose filter/screen device.

6 VOLTS/POS. GRD. NW INDIANA

"Yellow premium coolant is less than a year old, and though already filtered thru a plastic coffee screen, it is still a very dark rusty-brown."

Who is the maker (and PN) of the coolant and why did you choose yellow? Do you have aluminum engine parts - intake - cyl heads - front cover?

If the coolant is now rusty brown, you will need a cleaner and complete flush or it will turn again.

____________________________

These old engine blocks are notorious for rust buildup, particularly towards the back cylinders; it can be one of the main causes of overheating. When an engine is torn down for a rebuild, the blocks need to be thoroughly cleaned to eliminate the rust; a coat hanger is often one of the tools used. This could explain why your green coolant turned brown within a year.                                             

There could be another explanation, but it might not be plausible. Are you color-blind? I'm joking, but years ago, I had a friend who could not differentiate green from brown. 

____________________________

Amsoil propylene glycol (yellow/70%).  I like it mainly for it’s sealing effect (leaky steel head gasket).  Block has been flushed, but unfortunately, rust is an ongoing process (especially since I run an open/zero pressure system).  Even if there was a way to clean the cooling passages down to bare, shiny iron, with air & the 30% water, new coolant would soon be brown again.
Upon reflection, I may have underestimated the age, and likely re-using it would be a false economy regardless.  I have to wonder how much rust the 70 yr. old radiator may have retained also, contributing to the discoloration.  IT too had been chemically flushed over the years, but how well does rust adhere to brass, and how much did the circulatory flow play toward dislodging accumulation.  Guess I’ll find out after the new radiator has been in service awhile. The hose filter will likely be adequate protection to keep it properly functioning (rebuilt/hi-po motor waiting in the wings will need max cooling).

6 VOLTS/POS. GRD. NW INDIANA

When I inherited my 55 Tbird, it still had the original radiator (12-54). Although it didn't leak, I removed the cap and could clearly see that the core was rotten. I replaced it with a US Radiator heavy-duty 4-row copper & brass radiator. At the time, re-coring it would have cost as much as a new radiator. Re-coring the heater core would have cost more than the cost of a new heater core. 

Block has been flushed, but unfortunately, rust is an ongoing process (especially since I run an open/zero pressure system).

OPEN/ZERO PRESSURE?

Depending on how much sediment (hardened from engine heat and caught in tight passages turns) is in the engine block, and as it keeps breaking down, the coolant will always be compromised (particulates).

You have a BIRD, correct?

____________________________

Maybe consider a cheaper cost coolant until either the system is cleaned or the discolor eases -

https://prestone.com/product/prestone-all-vehicles-antifreezecoolant-ready-to-use/

Just make sure whatever product you choose is aluminum/iron compatible.

____________________________