The palette
Blond sandstone for the classical terraces of the centre, brick and slate for the rest. The real colour decision is the steel: the bridges are the subject, so give the arch a value that cuts against the dark ribbon of the river rather than sitting quietly on it. The common at the top wants a pale green, almost grey, with nothing in it, because emptiness is what it is for. Render the fall towards the quayside by tightening the weave of the streets rather than by shading; on a plan, a slope shows as crowding.