Here’s an interesting fact I learned today: any knit fabric that you find that has this speckled look is a is made from rayon, polyester and cotton (hence “tri-blend”). The variation in color occurs when the cotton (natural fiber), rayon (semi-synthetic) and polyester (synthetic) soak up/reject different amounts of the dye.

Also, if you’re buying a decent amount of knit for production, the vendor is likely to setup the yarn and run the exact weight and material composition that you specify.