To make the scene less grey and boring-looking, we'll give the scene a yellow tint as if sunlight was illuminating the city. Right-click below the two render passes you added, choose New and then Solid (Fig.07).
Then, under Solid Settings, give the solid a light, yellow colour (Fig.08).
Change the blending mode for the solid to Multiply (Fig.09).
As you can see, this gives us a slight problem as the sky just turned yellow too. This is because unlike the rendered passes, the solid does not come with a built-in alpha. To add one and get rid of the yellowness, first drag the matte render pass into the time line. Drag it to the bottom of the stack (Fig.10).
This seemingly cleared up the matter at once as the sky instantly went back to black. However, this only happened because the underlying matte is black in the area representing the sky and that anything (including light yellow) multiplied by black becomes black. To make the sky area of the solid truly transparent, you need to right-click the solid, choose Effect, Channel and then Set Matte (Fig.11).
In the Take Matte From Layer drop down, select the matte which will be the fourth option. As the matte doesn't come with an alpha channel, you have to change the Use For Matte option from Alpha Channel to either Red Channel, Green Channel or Blue Channel. As the matte is greyscale, the red, green and blue channel contains the exact same data and it doesn't matter which one you choose (Fig.12).
The solid is now transparent. As we'll be needing to apply the matte to other layers later, select the matte in the Effect Controls window and press Ctrl+C/Cmd+C to copy it. Next up is the Z-Depth pass. This can be used for two purposes, but here we'll use it to decrease contrast further back to put some "air" between the foreground and the background. Drag the Z-Depth pass to the top of the time line (Fig.13).