I don't prewash anything. I've been sewing for years, and never had any issues. If I ever do have an issue, I might change my mind, but I have so much fabric...I'd have to wash, dry, and iron for a few years to get it all done. Lol....that's not happening!!
I've even done bright batiks with white backgrounds...no issues.
If you are concerned, you can take a little piece of the fabric and put it in a bowl of water to see if the color comes out at all. You can use color catchers too when you wash it. I have them, used them a few times, but found that our clothes bleed more color than any of my quilts have.