I tried some brief experiments with some green and blue LED's desgined for instrment displays. The one thing that really surprised me is that the VC paper was nearly unresponsive to the green LED. Even making a tiny print, though a very thin negative took a very long time. In fact the paper was so insensitive to the green LED's it made me wonder if they would make a good safe light (I currently use red LED's for a safe light).
I notied that the color of the big bright 1W and 5W green LED that Huws used was different than the ones I was experimenting with (peak emission frequency was different). I suspect that this makes all the difference in the world.
The blue LED's required very short exposures .. but only got me to about grade 4.