I have only seen brown-dead ones used since that is all that is normally available. It would damage a tree to use green ones. If you are pruning a pine tree, it is best to wait until the needles dry and fall off before using them.

Caution, don't put the mulch up against the stem of the plant. This encourages voles to chew on the bark of the plant in winter, killing it. Keep the mulch back a couple inches from the actual stem of the plant.

Here is a link that might be useful: How to care for rhododendrons and azaleas.

Thanks for the info rhodyman