Formation of low and flattened cumulus clouds at sunset (hence their yellowish coloration) due to weak convection currents because of some surface heating from the Autumnal sun which at that time of year is too weak to cause much instability all by itself. Above these clouds, there were also stratocumulus opacus clouds from collapsed cumulus. The weather sounding on the second thumbnail depicts all of this explanation. The first thumbnail shows that the topmost cloud layer managed to produce some light rainfall hence a rainbow on the eastern side of the sky.