I found some copper mexican 10 and 20 centavos coins km#430 & km#437 that were in an old metal box, it seems that they were exposed to water and now they have a pink-orange color that is not very attractive, any suggestions of how to return them their natural brown color?

Thanks

There's no way to reverse chemical reactions on bronze coin surfaces. Any reaction you could induce to change the appearance of the current state of the coin would leave it with another unnatural looking patina.

I know people that would dip it and then retone it
But I guess only 10% of the coins turn out nice and believable enough
and this is from experienced coindoctors
So I guess for copper and bronze too bad but leave it as it is