A monotone chao is a very special type of chao. It is all one color, but unlike the pure ones you can buy at the Black Market, if you pet them with a dark character, they will turn a deep pink color. However, petting them with a hero character doesn't make much of a difference in color.
So far, I've only had white monotone chao, so I don't know if, say, I pet a purple chao with a hero character, would it turn white, lighter purple, or nothing? I'll try it this week so I can relay the info back to you.
How do you get one you ask? It's pretty simple. Just get a white colored chao from the Black Market. (I'm saying white until I can get more info on purple/yellow, whatever other color monotone chao) Anyway, raise your pure white chao to adulthood, then mate it with a normal adult chao.
To easily check to see if you have a baby monotone chao, first:
1.) Does the baby look pure white? If it is, go to step number 2
2.) Pet it with a dark character, does it turns a deep pink color?
If you answered yes to both of these questions...
CONGRATS!
If you didn't, hatch a new egg and go for it again.