If your sheets are white, you should wash them in warm or hot water to help lift stains. Higher temperature washes help remove stains and wash away the oils and dirt that cause yellowing. Just make sure to wash whites separately, especially if you wash them in warm water. Higher water temperatures can remove a little dye from colored fabrics, and if there are whites in the wash along with the colors, this dye can transfer to the white fabrics. This may leave your whites spotted or even dyed pastel colors. Also, be sure to check the label on your sheets for care instructions. Cotton, silk, wool, and polyester all have different care needs, so the fabrics your sheets are made of will dictate the temperature they can tolerate.