Women (here I mean XX individuals) have two different alleles present for each of the green (OPN1MW, also the OPN1MW2 duplication) and red cones (OPN1LW), since these are found on the x chromosome. X-inactivation means that only one gets expressed in a particular cell, but this means individual photoreceptor cells can express either allele. The individual proteins and gene encodings of the cones can differ, and small variations shift the spectral sensitivity to slightly shorter or slightly longer wavelengths. It's possible, then, for a woman to express as many as five unique-ish cones in theory -- though there's only been one 'true' tetrachromat found so far. Still, having red and green cone variants that respond with a peak preference shifted 10-20 nm in addition to another unshifted cone (or, better, shifted the opposite direction) provides a biological basis to expect women (again, specifically XX individuals) to have finer color differentiation. This explanation, however, could not occur following a hormone replacement.