[Blaaaaaaaah]

Grey= no exp I think ( really easy)
Green=Little exp ( easy)
Yellow= More exp than green ( medium)
Orange?= I think there is orange, more exp than yello ( hard)
Red=More exp than orange but probably will one hit KO you( Impossible)

Don't take me word for word but it is something like that

That sounds right. They have a similar colour coding system like ROSE Online.