your color changes will look lovely...I used the same technique for a rug I crocheted and I love the way it looks...I'd add a pic, but it's kinda worn and dirty now and needs to be washed.
as for needle size, 15's should work well, but it might leave your resulting felted fabric a little floppy. If it turns out like that you can always felt it again

that's the beauty of felting...it's VERY forgiving.