From what I've read, there is no way to stop the leaves from eventually losing their green, and turning either brown or cream. I wish there was.

you can preserve them by soaking them for several days in a glycerine solution... or press them for only a day or two, then seal them with poly-clear or something while they're still 'fresh' but flat...

I used this to decoupage ginko leaves over a carboard box- just used Modge Podge, in fact, and they're holding their color really well.

I suggest only pressing DARK green and silvery leaves. They seem to hold their color the best. The backs of wild blackberry leaves are a beautiful silver shade and many of the artemesia, dusty miller leaves work well.