Best way to change the background color for an NSView

Yeah, your own answer was right. You could also use Cocoa methods:

- (void)drawRect:(NSRect)dirtyRect {
    // set any NSColor for filling, say white:
    [[NSColor whiteColor] setFill];
    NSRectFill(dirtyRect);
    [super drawRect:dirtyRect];
}

In Swift:

class MyView: NSView {

    override func draw(_ dirtyRect: NSRect) {
        super.draw(dirtyRect)

        // #1d161d
        NSColor(red: 0x1d/255, green: 0x16/255, blue: 0x1d/255, alpha: 1).setFill()
        dirtyRect.fill()
    }

}