I feel your pain. Kooks are a beautiful wonderful series that probably falls into at least semi-numismatics. When you put together a full set from 1990 to present...it is definitely not bullion only, but a coin collector item. The dark toning will possibly affect the value (although there are toning fans out there).
I have a complete collection of kooks that demonstrates some rim toning on the early years-- but I rescued it from a Littleton album. How are you storing yours? If it is in an album, I would switch to airtites and some way to display those. But that is just my preference.
I collect a lot of Australian bullion coins (every bullion series, carded and uncarded). One thing I love about them is that they do not discolor very much at all. At least not mine. Except the ones I bought in an album, as mentioned already. None of mine bought new and stored in capsules have yet to tone.





















