Helen Allingham’s The Staircase, Whittington Court, Gloucestershire is probably not, as I’ve seen it labeled, simply a watercolor.
Watercolor, a highly translucent medium, cannot be made lighter by adding paint; highlights must be “reserved,” either with a wax resist or simply by avoiding the area.
Here, though, the woman’s dress appears to be painted in white over the shadowed wall, using the beige as the shadow of the fabric. The wood is more ambiguous, but does appear to have its highlights painted in.
Thus I suspect she also used gouache, which is basically just watercolor with something mixed in (often chalk) to make it opaque.
In any case, the fond little confrontation between a cat and her human would delight in any medium.