To make the dragon ferocious an option I think of right now is to use dark colours. As for the eyeball I wouldn´t give it too much bright, I´d shade it almost full black, but not all of it as it´d become emotionless. Maybe start form full black in the center and softening. And finally to leave two white spots close to the eyeball border one at the buttom and one at the top to make it more lively (because every eye reflects the light

However regarding to the use of colours I can´t give you advices :/ Personally I don´t know how to use them.