Herein we report experimental and theoretical analysis of the optical properties of planarized self-assembled colloidal photonic crystals confined within rectangular microchannels. A detailed mapping of the optical features, performed by microspectroscopy, is presented, providing evidence that spherical colloids confined in microchannels crystallize as single-domain colloidal crystal several hundreds of microns long with a very low concentration of intrinsic defects. A study of defective colloidal crystals is also made to illustrate this point. The effect of the different parameters affecting the reflection properties of these structures, such as crystal size, orientation, and nature of the substrate, is also analyzed. We show that the well-defined geometry and dimensions of the channels serve to template colloidal crystals with precise size, shape, and orientation, which imply accurate control of their photonic crystal properties.