To achieve a saturated blue or a vibrant green I use natural indigo. Indigo is a plant and the dye it produces, like most things in nature, is a bit finicky. Very occasionally when knitting you may find that your hands or needles develop a blue stain (easily removed with soap and water). This is caused by small particles of indigo breaking free from the yarn due to the mechanical action of knitting. “Crocking” is a natural part of using natural indigo. It will not affect your finished garment.
For more information about the why, how, and extent of crocking, please read my blog post on the subject, here.