Davina, I can’t see how even oxidized type could change ink colors. I believe it’s more likely that the type was insufficiently cleaned before its next use with a light color. What cleaning agents are you using? Is it possible that ink could have remained in the counters of your sorts and contaminated the color in that way?
Another possibility to consider is how clean your ink distribution system is, both disk (if using a platen press) AND rollers. Rollers are sometimes very difficult to clean and darker colors can hang on half of forever no matter how hard you try to clean.
Next time you intend to print light colors, dampen a rag with type wash, the real stuff. Poke around the counters of your type and make absolutely sure it is totally clean. Then check your ink disk and/or rollers in the same way. I suspect your light colors will come out much better after those checks and further cleaning, if necessary.
I hope I haven’t insulted your cleaning abilities 🙂 — but really, it can be really difficult to remove 100% of the old dark ink, and I know that from experience!!
Are you mixing your color with opaque or transparent white ink? This is often a problem with an opaque base, so if you can mix to your PMS swatches with transparent instead, that will take care of it. Otherwise, allow for a large overrun in each run, as the problem fades as you go along.