Here is a nice example by Juan Camilo Alcaraz using the free version of Unity3D.
The ease of changing the colours in this case is something that any user would be able to do. Though they are a little trapped in how they choose the colours, it is a great way to explore options with the impact visible in first person instantly - not via a rough plan and some colour swatches. Juan used his skills with 3ds Max to bake in lots of the lighting and fidelity before bringing the assets into Unity, this has lent the free version a quality that often lets down this engine, but it is looking good here.
Note you can try the demo using the browser plugin!