Brown Frothy color in radiator

[72tutone]

New radiator, water pump. I do not have the heater hose hooked up at the moment. The motor was just installed not that long ago. However, prior to that it sat for almost a yr while I saved up some $$$ to build it.
Could this just be left over sediment, crap in the block? It almost looks muddy.
Fearing for the worse I pulled the dipstick and its straight oil so that makes me a feel a bit better.
Whats the best process in flushing this out? Ive heard everything from freeze plugs removal or plugs in the block and I can open up.
Ive also heard I can remove the thermostat housing and crank a hose through the manifold and have it puke back out of the radiator cap.

Any ideas?