Statement from Maco Hamburg/Germany:

Rollei Digibase C-41 Kits contain 3 color developer (Part A, Part B, Part C).
Only Part C is sensetive to oxygen.
The original color of Part C is yellow.

After partial use the remaining liquid needs to be protected from oxygen either by adding protective gas or by transferring the liquid to smaller bottles.
When turning from yellow to red this particular developer part is no longer applicable.

These instructions are part of the product specification provided by MACO:

"The very high life period of the original concentrates is min. 3 years.
(Retention samples, produced 5 years ago, are still working well)
The only exception is color developer Part C.
The life period of color developer Part C is 2 - 3 years, depending on the storage temperatures.
The coloring of the Part C concentrate has to be yellow.
If the coloring changes into red, the concentrate does not work anymore.

In this case, you can buy single replacements for Part C:

#RDC05FC 100ml concentrate
#RDC01FC 1000ml concentrate."

With kind regards,
Sebastian Junghans