over time, the white rubber eartips that come with the ER-4 earphones, begin to yellow, even though I wash them every day in mild dish soap and water, and continuously wipe them every time i take them out of my ears with a moist napkin. are there solvents out there that will get the yellow out ?

try putting them in some fresh fizzy Coca Cola Noharm in trying and it might just work
alternatively use one of those toilet bowl cleaners they might work as well.
silly suggestion but you will notice they work well

i'll give it a shot....see if it works, but the bowl cleaner i have is clorox gel, wouldn't clorox use make the eartips crack after a while ? I spoke to etymotics a while ago and told them about using isopropyl alchohol and they told me that after a while it would begin cracking. not that i have a problem picking up a new set once a year, they are relatively cheap. the problem is leaving them sitting around @ my desk with that yellow look, people will get disgusted i think....

have you tried just normal detergent some of thatuff or st oxy action some of those spray on stain removers.
they might work

recently i let my white eartips soak in a little bowl of isopropyl for two days, and i was really quite suprised by how new they feel now. they're also very white and look brand new.

why not just dye them black?

i'd say a bit of H2O2 (hydrogen peroxide) would do the trick, but i'm a physics major, not a chemist, so i wouldn't trust me. i do remember reading it somewhere, though...