This is a difficult task. I try to use dark leaves and those with silvery colors. I have color-enhanced leaves with a mix of watercolor and a drop of dish detergent added to prevent the paint from beading.
Oregonlam.com sells UV protectant laminating sheets which help some, but leaves still tend to fade.
Check www.wwpfg.org for lots of pressing info.
If you find the answer, please let me know!!

Here is a link that might be useful: World Wide Pressed Flower Guild

Although there are lots of exceptions, in general it helps to pick your leaves in early spring, just as they open.

Unfortunately, that's also when many of the best flowers are blooming so time becomes an issue if you press a lot. If you can't press enough foliage in the spring always try to get the newest leaves.

Easiest way though, since you're already using Koch's systemic dye would be to skip the glycerine altogether or if you don't mind doing some experimenting use only tiny amounts of glycerine. Make sure you use a hydrating solution.

Koch's manuals are excellent, but they aren't targeted toward pressing. If you'd like to discuss this further please feel free to contact me and we can compare notes.

Here is a link that might be useful: my contact information