I don't do a lot of pressed flowers, but I have had some of my dried flowers turn brown when the humidity was too high. I also had some blooms stored in metal tins go brown;I use plastic ones now, & it hasn't happened since. Maybe some silica gel in your storage containers would help.

Thank you very much for your advice.The strange thing is I have done this exactly the same as I always would have done and got a different result. I think I will try your suggestion with regards silica gel but actually put grains/dust in along with the flowers I'm pressing. Has anyone else tried that previously? If so what were the results?
Cheers Emma

Emma, what are the 'grains/dust' you used with your flowers? I was interested in hearing if you had figured out what caused your flowers to turn brown. The only time I've experienced this is when I've tried pressing something like Azaleas which hold so much moisture. I usually dry/press mostly foliage or tiny weed flowers so everything goes in phonebooks and I don't bother changing papers...I just make sure there are several pages between each addition.

Just a hobby for me, though I'd still be devastated to find brown instead of nice colors when I at last peeked...usually weeks later. Do you suppose the grains/dust (?) somehow attracted moisture? I know with silica gel you must seal airtight the box after you add flowers.

The only other thought I had was that I reuse my phonebooks unless I notice a musty smell and discoloration on pages. Were you perhaps reusing blotter paper which might have had mildew spores (maybe not even visible to naked eye?).

Just some thoughts...I hope you'll post if and when you figure it out...thanks. josh