Its that easy.
But why does this happen?
Because, Boric Acid is a boron containing compound... Any boron-containing compound will cause flames to emit a green colour.
When boron compounds are heated, electrons absorb a certain amount of heat energy that causes them to jump to higher energy levels. After some time, the electrons lose this energy and fall back down to their original levels, emitting this energy in the form of light. Because the energy absorbed by electrons is different per element, each element will give a different colour.