Actually my problem is tone, it's not about what kind of media!
so exciting! My Genius Professor Martin simply and directly pointed out my problem by a quickly glance.
He kindly copied a page from my sketch book and gave me that told me to focus on tone, plan how to paint it.
It's amazing! I felt I finally realize what's wrong in my previous image.
lovely practice, I will keep playing it ~<3
Martin gave praise to this one :)