Barry, you might try a dilute bleach bath to see if it gets rid of some of the yellowing. I have never tried the new cyanotype formula, so no idea how it would react in the bleach bath. Actually, with cyanotypes, it's hard to predict how they will come out period.
Me - I have gotten to where I like the "cyan" color of the cyanotypes and rarely try any toning. The natural color has kind of grown on me. But every now and then the temptation to tone one crops up and I use a dilution of 1 part liquid household bleach to 3 parts water. I do it right after my print has washed. I slip it into the solution and I watch closely since it doesn't take very long to see a reaction, then I pull it out and rewash for a bit then dry. I usually get a mixture of colors, but because I am using bleach I don't seem to get the yellowing of the paper:
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