There is something I'd like to share with you. Don't know if anyone approached post-processing in this way before and would love to hear some opinions or even make others who can do it better interested in showing what can be done with it. I browsed internet for a very short time to see if it there are Photoshop tips about it already, but my basic keywords didn't return anything bar photographic depth of field.
Long story, short, you know that colder tones are perceived as further away and warmer tones are percieved as closer to the spectator of the image. It can especially be seen in classical paintings. It can be used in landscape or any other type of exterior renderings where you want the focus to be on the foreground. Or even change the game and make the focus on the background by switching the warm-cold locations.
This is how it goes:
Render out an image with depth pass. Paste it on the original image in PS as a separate layer. Go to adjustment layer and choose curves. Change the blending mode to color (so it doesn't affect your lighting, just the color!). Make adjustments to emphasize red or magenta or yellow (or combine all of them for the best effect) and paste the depth pass into this newly created layer mask by:
1) copying the depth pass
2) click+alt on layer mask of curves, it will open the mask only.
3) now paste it
Do the same thing again: create another curves adjustment layer, this time make cold colors emphasized (cyan, green, blue). Again, in the described way paste the depth pass into the curves layer mask.
Now you need to play. To be better at this game, know that where the mask is darker it means the curves will have less effect for each respective layer. On the layer whose toning you want to affect closer objects, you need to invert the depth map (image -> mode -> invert) so that whites are closer and blacks are further away. Make brightness and contrast adjustments by using levels on each mask (don't forget to hold alt while clicking the mask! Otherwise, you'll be changing the nearest active layer).
Here are examples of some of the many paintings that utilize this technique and my own post-processed renderings.
These are my old interior renderings, so it's not the best way to showcase this ability, but you'll get what I mean.
I'm also planning on making a tutorial on this technique with a different render.
(first render - nothing; second render - cold tones are closer; third render - warm tones are closer).