The chemical reactions and pathways resulting in the formation of the phytochrome chromophore. The phytochrome chromophore is a linear tetrapyrrolic prosthetic group covalently attached to the large soluble protein phytochrome. Light absorption by the phytochrome chromophore triggers photoconversion between two spectrally distinct forms of the photoreceptor: Pr, the red light absorbing form, and Pfr, the far red light absorbing form.