The Blender manual page says about Diffuse direct and indirect as follows:
Diffuse Direct
Direct lighting from diffuse BSDFs. We define direct lighting as coming from lamps, emitting surfaces, the background, or ambient occlusion after a single reflection or transmission off a surface. BSDF color is not included in this pass.
Diffuse Indirect
Indirect lighting from diffuse BSDFs. We define indirect lighting as coming from lamps, emitting surfaces or the background after more than one reflection or transmission off a surface. BSDF color is not included in this pass.
What I expect from lighting passes is a gray-scale image, or RGB image with color reflects the light source's color. However, the Diffuse direct/indirect images made by Cycles gives weird colors and I cannot explain it.