You can use this (common) methods in your 3D modeling package or game engine to achieve the desired effect:
1. create several light sources with different parameters (colors for example) and randomly spread them over terrain or around model (too many lights will hurt performance, so you must figure out yourself their amount, depending on whether it will be used in real-time simulation or not).
2. Create second layer with other texture and blend it with first texture (also in random places), maybe using some mask texture, or use "texture paint" (refer to your software documentation for the details); use different levels of transparency to get smooth transition result.
3. Rocks and similar objects can be rotated, scaled to get some randomness too.