Question

How to describe a mask

Answer 1

Describe what you see, for example, "that young girl over there has a beautiful midnight blue mask with white and light blue sparkles on it." 

Answer 2

Describe the physical appearance, of course, and maybe say what the appearance makes you feel. I only say this because many historical theatre masks were supposed to display emotion, and give the mood of the character or scene. For example, "The bright colors mixed with the cheery smile of the mask upon her face to create a feeling of immediate joy that brought a halt to his current emotions."