 Solution

The first zero of each colour is at $\theta =\lambda /a$. Blue therefore reaches its first zero closer to the axis than the reference green wavelength, and red reaches it farther away. At $\theta ={\lambda }_0/a$ the reference colour is absent, while other wavelengths remain: there is no common black first minimum in white light. Adjacent regions change hue as different colour minima pass through them; wavelengths longer than the reference are still in their central maxima and tend to dominate near its first zero. The exact hue depends on the incident spectrum and spectral response.