Stuff in brake fluid?

About brake fluid... If a container is opened and only a portion is used, The remainder is often left on a shelf for months or even years. Since the seal was broken the fluid can become contaminated and should not be used.

About new parts... New cast iron parts, including master cylinders and calipers, have a light oil coating applied to prevent rust. The oil will discolor the fluid but would not cause any other issues like soft pedal.

You could flush and pressure bleed until you have clean clear fluid all around but guess what ? as the linings wear the rubber cups and rings move across oily areas and pick up small amounts of oil again.
Since the fluid isn't circulated that oil and discoloration will remain in the caliper/ wheel cylinder and never be seen until next bleed.
Still, none of this will cause poor braking.

For the record... Replacing calipers should be both at the same time.