What are the main ingredients of an Espresso Martini Cocktail?

Mainly of a Liquor, fresh Espresso and optionally some sugar.

What is the color of an Espresso Martini?

Rich, dark and creamy

What Espresso should I use?

If you use an fresh Espresso you might get some more foam and flavor. But always be aware the you better mix the hot Espresso with cold ingredients. Othwerwise you dilute your drink to much.