Metallic head and front of body and motility of melanosomes within the cells on the body (the melanin/black pigment can move within the cell).
Best seen on green/blue/purple Moscows, the body color can lighten or darken. The color of the fish is not a single layer, there are multiple layers. It is normal for them to change color, it's not just the lighting. They can be bright, dull or dark depending on mood and conditions.
The tail may show unexpected bands on the trailing edge as the males mature and the pigment cells develop.
What is called a black moscow does not have this color change, they should always remain solid velvety black, Seems to be an additional layer of black in which the melanin cannot move. Inferior fish may not have complete coverage of this layer and may show a little metal on the head/front of body and some color in the body or tail in normal bright lighting. This really can't be evaluated until the fish is mature. Flash photography tends to show a hint of color in even good black moscows.
While most moscows available are solid colors, this is not part of the definition. There are patterned (snakeskin type) moscows.