There is provided a color wheel including a plurality of color filters arranged so as to form a disk, and a retainer member disposed at a center of the disk and formed into a C-ring defining an open mouth portion through which an area of one color filter is exposed. Also, there is provided a color wheel assembly which incorporates the color wheel described above, and which further includes a motor to rotate the color wheel, and a rotational position detector to optically detect a rotational position of the color wheel. Detection light emitted from the detector is set to fall incident on the C-ringed retainer member while the color wheel is rotated thus intermittently impinging on the open mouth portion, namely on the one color filter, which causes the detection light reflected thereby to come out with two different intensities that are sensed by the detector.