Filling the array with known non-zero values will help find implementations which aren't spec compliant. For example, consider a GPU which does some sort of modulo operation during the memory fetch, but the modulo is aligned to a larger memory boundary than the size of the array, so it fetches values within the array but picks up (b, a, r, g) from adjacent elements rather than (r, g, b, a) from a single element. Basically, I think that verifying the spec as precisely as possible will reduce the likelihood of bugs in the future.