Best Of

I agree on color change. Did a Rhomar exchange (just replacing original Rhomar with new) and noticed almost zero color change.

I agree with @ScottG.  Replacing old Rhomar with new you will not see a color change to know if the old has been replaced with the new.  Color is a good indicator if you are replacing the old Century with the Rhomar as one is yellow and the other is greenish.  As Scott mentioned I would just push in a bit more than your system requires and call it good.  Since the color change is not going to be noticeable I would also recommend using a shop vac to suck out the old fluid at the end of the line as you pump in the new.  It's the method I used on our 2021 400 and I think it just helps in removing the old and limiting the mixture of old and new glycol.  

Ditto what @bergger said for us. It has never tightened. I just snug it down.

@Greenhil, I doubt you will bbe able to identify old vs new glycol by a change in color, or pH. If you are simply pushing the old glycol out with the new, I would go by the volume of new fluid pumped in. To do this, you would need to determine the approximate total capacity of your system. To be safe, I would probably pump in an additional 10-20% above that volume.

Just did mine in our 400. Had the part for quite a while. Also moved the sink water connections up so those are out of the way as well.