The two waves are out of phase for observation angles different from zero due to the added path length for one of the two slits. Then repeat this for another colour. The double slit interference can be used for spectroscopic analysis as the diffraction angles are different for a given wavelength. In a diffraction grating one usually uses more than two slits to create a sharper interference pattern. A light front impinges on the entire aperture with an oscillatory electric field present along the entire aperture.
Spherical waves emerge behind the two slits and create an interference pattern.
At large distances from the aperture they can be approximated by two plane wave fronts emerging from either slit. By adding several light fronts with respective phase delays as in the two-slit case one can show that the constructive interferences occur in narrow regions, i.e., one can observe sharp spectral lines. A similar analysis can be carried out for a reflective diffraction grating. For certain angles constructive interference will occur. You can carry out a spectroscopy lab at home.