Red. My DC's have attended schools with all manner of uniform colours, and red discolours least quickly IME.
Pale blue is a righteous PITA if you have tall DC's, as once they get past size 10-11, they are only sold in M&S and Next...
(5ft2, 11yo, Y6 DS1 is currently in age 15 pale blue polo's, and I can seemingly only get that in Next.)
Next pale blue polo's wash and wear the best. Tesco ones fade really quickly and go bobbly, Asda ones go all baggy and misshapen after a few months (usually just after their 100 day thing, which is actually 100 days after purchase, not 100 school days...). Sainsbury's ones are OK, but come up quite slim fitting, and do fade, though not as quickly as Tesco ones...
Next ones last and last and last! My DS1's hand me downs do DS1, DS2, my friend's eldest DD and her DS, and still look like new! Well worth the extra cost IMO.