Canonical arithmetic progression dichotomy

Every colouring of the positive integers, even with infinitely many colours, has an arithmetic progression of any prescribed finite length on which the colouring is constant or injective. Encode a progression by the equivalence relation of equality of its colours. This gives a finite colouring of its starting point and step. A Gallai theorem for an integer lattice copy of a suitable two-dimensional pattern makes either all indices inequivalent, or one equal-colour pair range independently over two whole progressions. In the second case, one of those progressions is monochromatic.