get some Oxi powder, take your gi collar and wet it, put powder on collar, take a medium bristle brush and scrub for a good 5 minutes. Wash on a medium temp i.e. 40 (or less) degree wash. The discolouration is usually elements from sweat and/or use of deodorants. If you can, avoid tumble drying, as the heat will just "bake" the stain in and make it impossible to remove.