Normally, all of Ceros are red in color, but why is Ulquiorra's Cero was green? Does it mean that it's more powerful? What makes it so different?
It is true that the default color for the ceros is red, but this color is usually used by normal Hollows or Menos.
But there is a o lot of Arrancars and Espada that use a different color of Cero, it is not just Ulquiorra, see this page here.
Most Cero are a crimson color when used by Hollows, Visored or Arrancar. While most Cero are commonly crimson, several Arrancar have shown various colors.
It also says:
The power, force, speed, and blast area of the Cero is dependent on the strength, spiritual power, and sometimes skill of the user
So, the color does not influence directly to the power of the Cero. In fact, the color changes depending on the user's personality and style. Ulquiorra's cero maybe stronger, but it is not related to the green color, but in Ulquiorra himself.