When white light or continuous range of colours of light is incident on a diffraction grating, the range of colours will appear at different angles . The angular separation of the blue (short) to red (long) fringe increase as the n increases. The angular separation at higher orders are larger. If I want to find number of complete orders that can be seen, I would need to find only for a larger wavelength(red) . If the largest order n of red can be seen surely the same order blue can be seen. You will surely get a complete set or range of colours of n orders if you just find highest order for the largest wavelength by setting sin 90' = 1 and rounding down the value n found using n=d/wavelength .
