Attached Link: http://www.microbion.co.uk/temp/tree.zip

Very nice tutorial. One thing to add: you mention that you can modify the material in two ways. There's also a third way which might prove very useful, and that's to use a clipping filter in the color production area. By starting off clipping, say, all above 10%, then progressing right through successive frames to clip all below 90%, you could animate a tree turning from green to yellow very easily.

There's a (very) short .avi for download at the link which shows what I mean - it's about 3Mb long, if you are on dialup.

Steve