The palette
Two rivers, one ink: the Schuylkill and the Delaware take the same colour, and together they frame the sheet left and right. Give them enough depth that the curve of the one and the straightness of the other read from across a room. The squares take the green of every playground and lawn on the sheet, so they stand out by their shape and their size, not by a colour of their own; a green clearly set off from the streets helps. Fairmount Park, in the top left corner, will be the largest patch of it.