They look the same material as the acorns i had - not saying they are the same stuff, just the way the dirt is ingrained into it
they will clean up with a good wash - you could try a cif or cillit bang stuff but be careful it doesn't eat it too much
if you want to bleach them - it will change the metal and it will need a polish ~ this will not look the same as it was
i also suspect they were a off white anyway or quickly changed after a while with use or shelf wait
hope this helps - cheers B