The strong colour of polydiacetylene polymer crystals is due to the backbone exciton. The energy of the exciton is strongly affected by Coulombic interactions with the surrounding crystalline environment. X-ray diffraction and optical absorption measurements of the blue phase polymer crystals of 4BCMU show that a change in the energy of the polydiacetylene backbone exciton can be related to a change in the inter-backbone spacing within the crystals. Over the range of inter-backbone spacings measured, this variation is in very good agreement with Coulombic screening theory. It is calculated from these results that inter-backbone Coulombic screening lowers the value of the exciton energy in the blue phase polymer crystals of 4BCMU by -0.362eV, which is 15.4% of the total exciton energy. Coulombic screening is shown to have an important effect on the optical absorption properties of other polydiacetylene derivatives. Calculations and resonant Raman spectroscopy measurements show that a combination of a low level of Coulombic screening and strain in the crystal lattice accounts for the red colour of the polymer crystals of PTS-12 and TCDU.