Don't think there is a quick solution, although those kept in the dark certainly go darker, markers & hands on safe queens are often very orangey/brown - but this is over many years. The opposite is that those exposed to light (worn or left out) tend to stay whiter.
I think Jed posted a comment that a mate had left a Comex on a window sill for 3 months in the summer to bring back a whiteness to the markers, again a bit of a long process.
Unless anyone has been experimenting with other techniques ?