What are some card in MTG that can change a creature's color?
There are a few cards that can do this. Some specifically turn creatures black:


Darkest Hour
Grave Servitude
Sinister Strength


Others let you choose a color, and make creatures that color:


Alchor's Tomb
Blind Seer
Distorting Lens
Illusion
Painter's Servant
Shifting Sky
Sway of Illusion
Tidal Visionary


And, finally, there are some cards that let you change the word "black" on Corrosive Mentor, so that it affects green creatures instead of black ones, for example:


Alter Reality
Crystal Spray
Glamerdye
Mind Bend
Sleight of Mind
Spectral Shift
Swirl the Mists
Trait Doctoring
Whim of Volrath


I've bolded all of the cards that are either black or colorless (so they don't restrict what other cards you can play). Across the last two lists, blue is the most common color, so blue is probably a good second color for this purpose.