Unique hues do not nicely coincide with hues that are invariant with intensity. This is at variance with theories that relate both to the zero crossings of opponent mechanisms. It is shown that, in the Walraven theory of the Bezold-Brücke effect which is based, both on diminishing return receptor characteristics and on zero crossings, the latter assumption is superfluous. That means that the Bezold-Brücke effect can be explained completely in terms of receptor behaviour and that unique and invariant hues have no direct theoretical coupling.