It's hard to describe what I'm looking for because I really don't know, the word 'organic' seems to fit. My game is 2D, I have a textured background with 3D rendered objects on top; I'd like something that brings it all together and makes the 3D objects look like they belong to the background environment and vice versa.
I've been trying a bloom technique that blends the blurred scene with the original using hard light and also overlay formulas (Photoshop equivalents); they look pretty good but I was hoping for something better. I wouldn't mind bringing out some more colour as well. I don't really know what else to try except a bloom-style technique so any ideas are welcome.
You could look at manual colour correction to match one rendering to the other. The Unreal documentation gives a pretty good explanation, and it's easy to implement yourself. The shader is almost a simple copy-texture, but you use the input colour as texture-coordinates into the LUT texture, which gives the corrected output. e.g. tex3D( clut, tex2D( input, screenUV ) )