Can I render just the Diffuse Color Pass in Cycles? I tried to check just Diffuse Color Pass, but it rendered the whole image anyway.
To the best of my knowledge, it is not possible to directly render only a single color pass in Cycles. However, after rendering your image, you can access the different color passes by using the compositor.
Before rendering, make sure to go to the Render>Layers menu and select "Direct", "Indirect", and "Color" for the material type you want to single out. This makes sure that you have all the necessary color passes when you get to the compositor.
In the compositor, instead of using "Image" as input to the Composite node, you want to combine in some way the three color passes "Direct", "Indirect", and "Color" for your material type. In the example below, I used two Color Overlay nodes as shown in my compositor setup.
My original image:
Diffuse Color Passes only:
Glossy Color Passes only:
My Compositor Setup:
(Ok, lets see if i can write a proper answer)
With this render machine you will have to activate the render passes of your choice (in your case the three under Diffuse) and in compositing you will have to combine them ((Diffuse indirect + Diffuse Direct)*Diffuse Color) together, sorry if this is not much of help, dont really work with cycles, i think Bartek Skorupa have some really good tutorials in compositing on Blender cookie for cycles, check them if you want a deep explanation :)
Edited: Sorry, i explined for the wrong render machine