Quote - Grey?  What, turn off all the textures in the city and replace them with a flat grey material???

Yup :-)

Quote - What sort of HDRI would I need for that?

Whatever gives you nice lighting :) but probably something that sort of matches the colors of your scene.

Quote - When you say overlay in PhotoShop, you mean paste that grey image into a new layer on top of the original and then reduce the grey one's opacity?  Yes?  No??

Adding it into a new layer on top, then setting the new layer to "overlay" or "multiply" its a bit of tweaking to see which will get the nicest looking results. Keep in mind, in multiply mode, anything darker then pure white will darken your image. While with overlay, anything darker then 50% grey will darken your image and anything brighter then 50% grey will brighten your image.

If you have a photoshop version that supports layer grouping, you could use the following layer setup to control the effect even better:

• Bottom layer is your main render
• Above the main render, you place a layer group, use the groups blending mode on overlay, or multiply.
• Inside this group you put:
     * the greyscale render, set to normal blending mode.
     * a levels adjustment layer on top of the greyscale render, also set to normal blending mode

Now you can use the level adjustment layer to adjust the white balance of your greyscale render, without 'damaging' the actual greyscale layer itself (meaning you can tweak it over and over without losing quality) and use the groups blending settings to blend the entire group as if it were just one layer.

hope that helps

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