"Monotone" Chao are the single coloured Chao purchasable from the Black Market.
For each of these Chao, there is a "two tone" version. i.e it has highlights of a different colour.
The Monotone Normal Chao is just the monotone version of a normal Chao.

From Shining Chaos Chao
"It has a constant colour about it, the whole body is of a single colour, however, the colour changes. Some, inexperienced, people say it changes at random, or has no particular path of colour change, they are horribly wrong. It's body colour changes like that of a normal Chao's colours."

"Every Chao has what I like to call a 'base colour', that colour is the one that usually varies the most between a normal Chao, and a half normal Chao, and that colour affects the rest of the colours; the Chao in question has it's whole body the colour of it's base colour, just like a normal Chao. But normal Chao has other shades of the base colour, although this one, does not."

"Recent observations have shown that the Chao's whole body is not necessarily the same colour. Every body part's colour is calculated independantly to the other. This means that the arms, legs, body, tail and head may be different colours. I have only seen this on neutral/swim Chao, most of them have tails and feet of odd colours." 

to get this chao you must   mate a normal chao with a jewel chao from the tiny chao garden there is a 50% chance you will get it.