The problem with that part isn't the discoloration (which is fine; they all will have that). It's with the lip at the top shearing off. That part is known as the 'sun shell' -- what the planetary gears, which include the 'sun gear', go into.
Anyway, when they shear (you hear a loud 'bang'), you'll lose forward and reverse gears (b/c they're just freewheeling at that point).
Since this is one of the known weak areas of the 4L60e, I'd recommend an aftermarket replacement. Sonnax makes one; another (more famous) one is called a 'Beast' (I have it in mine, but I had mine rebuilt by a trans specialist). They (did) run about $80. Money well spent for the peace of mind.
Oh, and I second those Transmission Bench videos. Takes awhile to get through all of them, but they're thorough. I'd recommend a viewing up to the point you're at, then watching the next 1-2 before you go further. That way, you have an idea of what to expect.
I just came across this thread, and didn't look all the way through it yet. But I can tell this is your first transmission rebuild. You're a brave soul, and to be commended for attempting this. (not meant to scare you away; you're 'committed', now)