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Atmospheric perspective is the effect you're talking about. In general, the further back something is, the more air/atmosphere is between it and the viewer. This makes the object have softer edges, less chromatic and cooler. (There are exceptions to this and every rule). The easiest way to achieve this effect it to make more sky color than you need and add it to the local color of the object that you want to recede. The more sky color you add, the further back the object will appear.