The palette
The real subject here is an angle: give the streets enough contrast with the background for the meeting of the two grids to read at a glance, and keep the darkest ink for the motorways along the river. The water is thin, a creek and a river drawn as threads, so it needs an ink strong enough to hold a narrow line. Green comes in a few large patches, the parks with their lakes; a mid-toned green gives them weight without competing with the grid.