Thanks so much for Mr. Fowler's teaching! It's just so straight to the point and attracting for a newbie like me.
The painting cost around 3 hours in total (which actually took too long) and I am quite happy with the result. Hope more details can be added in later.
I arranged the color pipeline by :
Analyzing the lighting --> Applying core shadow color --> Using a large brush to define large areas --> Using a small brush to make dots for detailed area pattern --> Making Transition for dots --> Highlight --> Using detail Brush and Smudge to smooth/direct the texture