I don't think you can really lighten them. Maybe bleach them until they lighten up, but that might be too corrosive for the fabric. You could try dying them white and then darken them to your color of preference. It's easier to dye darker, but not lighter.
Of they are polyster blend you could try a very very light acid wash but that will make them very transparent. The acid will eat all the cotton off and strip the polyster of color you would then need to wash it with a box of baking soda to neutralize the acid.
All of this work it would be easier to get the correct color or a light cotton suit and dye it.