In case anyone is wondering if I did cleaned my white earphones, here's how I did it.
They are not 100% white, but definitely a lot whiter.
First I tried to brush them with toothpaste as suggested by @zzzzz , but I didn't quite see any plausible results, then when I was tired of brushing them and as a last resort, I sprayed them with bleach (that liquid that makes your clothes whiter) and brushed them again for another 2/3 minutes.
I wasn't seeing any results, but then I sprayed them again with bleach and left them outside in a sunny day so they could dry and you know... I just sprayed them with bleach, they are all soaked, so I want them clearly to dry :) . After a few hours being exposed to the sun, they started becoming whiter. At the end of the day, the results were pretty good.
Now I don't know if it was soaking them with bleach and its interaction with the sun, or the combination of the toothpaste then the bleach plus the sun, but they definitely became whiter.