As long as the Lobes do not have any large areas flacked off or large areas worn through the hardening that they are wearing adversly they can be reused.

They Followers are going to rinde on the highest spots so some rust pitting on the lobes is not a big issue; with the exception if the pitting is real deep right on the main wear area on the tip.

Where I used to work we would get some 180 Grit Emery (sometimes called wet-dry Sand Paper) and we would work with it on the Camshaft Lobes in a patteren that is about 45 degrees if the direction of the wear pattern that is already there.
If you do that from each side of the Lobe it leaves you with hundreds of Xed patterns acrossed the Lobe.
After that you can follow up with 300-400 grit emery paper.
I forgot to mention that whey you do the above you use light Oil or water.

The above process helps to break up the wear pattern and gets rid of small pitting and rust without altering the demenions.

Since we were rebuilding the Engine and the Camshaft was out we did not have to worry about getting any grit into the Engine.

The same should be done with the Cam Follower wear areas.

From what I have read at some point in time they replaced the Camshafts with ones that had harder Lobes and Followers that were also harder.

I would not worry about the discoloration.

As far as using the Camshaft with Lobes that are worn a little rounded but not pitted or flacked. They can be reused but you will lose a little bit of your Valve lift/opening. This does not seem to be much of an issue on Diesel Engines; unless of course the wear is severe.

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84 300D, 82 Volvo 244Gl Diesel