Cycles always renders RGB channels to the same values under sun lamp? I'm rendering a ground surface that "catches" the diffuse reflections from a blue sphere (earth). While I was using a point light, the rendered image is indeed blueish. When I switched to sun lamp, everything looks grayscale.

I've ensured the render output to be RGB, but my rendered image has always the same RGB channels as if it were grayscale.
I figured out why. The reason is that my lamp size is too big, which leads to almost no shadow on the shadow catcher.

So my region of interest is actually illuminated by the white light source (instead of being illuminated by the diffuse reflections). With a grayscale diffuse map, the whole thing renders to grayscale.