The addition of blanched, puréed spinach (or other greens) to the dough is the traditional method for coloring fresh pasta dough green. Green pasta is used in some lasagne.
The spinach may be chopped by hand, to produce a green-speckled dough, or it may be processed to a purée, for a homogenous color. If hand-chopped, the spinach must be very finely chopped or the dough won’t hold together. In both cases, it’s important that the spinach be squeezed out after cooking so it is as dry