I love hunter green and aqua blue on greys. Also, a deep greyish purple matches a grey very well. I also like getting not just saddle pads but new tack like boots, a new halter, or a new bridle. Get him his favorite treat and spend a lot of time with him. If your barn has a trail, go on trail rides with him and do all his favorite activities. I hope your pony gets to enjoy his birthday!