I know it's hard to tell from the poor pictures but this method really brings out colour and removes yellow from old Lego! (You can see it best on the 1x2 grey brick) The 2x8 plates are in good condition for comparision.
Method; take bricks add one scoop of oxy-type stain remover and enough 30% hydrogen peroxide (I 'borrowed 'this from work but I think pharmacists sell it) to cover and leave in the sun for 2 hours.
I was amazed at the results, however it may make the bricks brittle with time but they seem fine now.
Inspired by this retr0bright.wikispaces.com/