I've had this 586 hanging around for a while and always wondered why these grips have a different hue from the rest of my guns. Then I saw a thread about rosewood grips today and wondered what these grips are made of. They do have a bit more reddish tint that the usual grips.
They appear to be standard factory Target stocks of Goncalo Alves. These vary in color; yours have nice contrast and variation, while others are very uniform and plain.
So, how do you like the rest of the gun? Hope this is helpful.