I am sorry but short of getting artsy by hand coloring those spots with fabric paint or sharpie pens there's really nothing else to be done because the color has been permanently removed. :-(
You cannot get the color back without dying the whole sheet, and even then, depending on the size of the bleach spots and the original color of the sheet, you may still be able to see the spots. Personally, for sheets, I'd just live with the problem. If it is just tiny spots, you could try coloring with a jiffy marker, but, again, for sheets, I wouldn't bother. If you want to dye them, get Rit dye, and follow the directions. The best bet would be to try to dye them the original color with Rit dye.
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