What is added to pasta make it yellow?

Durum wheat is a type of wheat that has a high protein content, which gives it a yellow color. The semolina is made by grinding the durum wheat kernels and then sifting out the bran and germ, leaving the starchy endosperm behind.

So, while there's nothing "added" to make pasta yellow, it's the inherent color of the durum wheat semolina that gives pasta its distinctive yellow hue.