I would check out usplastics.com. I was on there a while back and noticed that they have plastic balls with threaded metal inserts of varying sizes. It might be worth a look.
Here's the
link.
They come in black only so you would need to paint them. Their customer service is pretty good and I would imagine they could even recommend a paint to you.
I've ordered from usplastics before and they are really good with customer service.
Hope that helps,