Found THE ANSWER recently. Have two boys - both plays Majors bball and one plays high school 2 - 3 times a week. ZEP now makes a spray called "Spot and Stain Remover". Available at Lowe's and Home Depot for $3.00 a can. You can literally WATCH the deepest and darkest of stains "magically" disappear as you spray the pants with this stuff. Then, wash with the hottest cycle possible. Pants look brand new and this stuff doesn't "eat" the fabric and make it pull / rough. (Iron Out DID eventually always eat the snaps on both boys' pants.)
Two words for you and just a little soap sprayed on beforehand.......Any kind, doesn't matter PRESSURE WASHER! I am amazed noone came up with this one. 5 minutes and they are ready to dry
I got a pressure washer last year for Christmas; gas powered, 2500PSI(dont bother buying one with less PSI)
I will lay a towel down on the Driveway first, then lay the pants on top of that........spray away...flip over and repeat My wife wears her rain boots when she does it. DONT Hit your toes with the pressure washer.......
BTW-My wife has tried all the things mentioned, settled on White bright, soaking for 3 days, etc etc...blah blah....The Pressure washer has made my sons pants look brighter than any other kids on the team. I would NOT recommend the ones at the car wash due to the fact that they use recycled water that may have sand/grit in it.....
I bet many of you already have the the pressure washer you need