White chess pieces have darkened - how to lighten them?

I’ve had this set since my early teens. The white pieces have darkened over time. I think this is a property of the wood since an elderly guy at a care home I used to go and play chess with as a kid had the same type of set and had the same problem. He bleached the white pieces but that was pretty drastic!

Any suggestions how to lighten the white pieces would be greatly appreciated!

The first thing I noticed is the knights seem to be darker than the other white pieces. The rooks show they are striped. So, any bleaching should be taken with consideration to this because wouldn't it still looked striped?

I can't post links but on Youtube, I found this "Yellow Wood Stain Colors and Polyurethane Using Yellow Dye Powder From Keda Dye With Polyurethane".

You can search it there and the idea would be to find something like the yellow stain formula. Maybe you can find something more between a yellow and white at a hardware store.

Ok, now I can.

Thanks for the suggestion.  Here are the darkest pieces. The wood has a dark grain in it that has become more visible. It may be olive wood.

And these are the lightest ones.

Place them out in 42 degree sun for a day (Sorry, it barely gets past 10 over there).

Thanks for the suggestion. I’m not sure whether UV light actually contributes to the darkening though by accentuating the dark veins. 

That's why I think you have to coat it with something. It won't have a wood grain look, but you can then more easily modify the color more consistently.