from the "Preparing and importing Photoshop files" section of After Effects Help:
"Layered Photoshop (PSD) files need to be saved in RGB or Grayscale color mode for After Effects to import them as a composition and to separate the layers. CMYK, LAB, Duotone, Monotone, and Tritone color modes are not supported for layered files; After Effects will import a file that uses one of these color modes as a single, flattened image."