Rotax Oil Elbow's

Sean, first, I think that the use of the elbows is a good idea, it allows the internal size to not be compromised by hose collapse from a tight bend in the line.  

Second, the metal is coated for corrosion protection.  The old versions were yellow in colour, a Zinc Chromate, but the chromium used is now banned in the EU so they had to switch to a replacement for the chromium and the resulting materials are now all silver.  (you can see this change over the past 10 years as all zinc chromate parts have been replaced with the new silver versions) This is not a decorative plating so it will oxidise after time and turn more a dull grey.  

The oil temperatures will lead to some of these bends changing colour faster than others.  in time they all will change but it is purely cosmetic and will not be an issue.  I suspect the ones that have turned somehow are getting more heat or exposed to more environmental conditions that cause the changes to happen faster.  

Cheers

To add to what RW said, the coatings are deliberately sacrificial.  The go-to for this used to be cadmium, but that’s banned in many areas.  Most are now zinc based and do become dull earlier than cadmium.  These coatings don’t depend on sealing the surface like painting or powder coating. They provide cathodic protection to the base metal and will even migrate to protect areas where it is scratched through. Essentially, the coating is specifically more reactive than the base metal and depletes itself to stop the base metal from corroding.  Just about the same as a zinc cathode attached to the hull of a metal boat.

If oxidation in one area of the item (say inside the tube) is higher, that will cause other areas of the coating on that same item to also oxidize faster. It has the ability to somewhat balance to oxidation across the surface of the coating.

Without the dichromate coating, you do notice the color change earlier in the coating’s life.  Environmental conditions affect the rate of oxidation so is some areas are a little hotter or damper, oxidation may be increased.