1. A target tracker adapted for a tracking of a target object in a time series of frames of image data, the target tracker comprising: a tracking object designation acceptor configured to accept a designation of a tracking object to be the target object in the time series of frames of image data; a target color setter configured to set a color for the tracking of the target object in the time series of frames of image data as a target color; and a particle filter processor having a set of particles movable in the time series of frames of image data in accordance with a prescribed rule, the particle filter processor being configured to apply a first subset of the set of particles to a first frame of image data in the time series of frames of image data, for a first set of measurements to determine a set of color likelihoods by comparison between the target color and colors in vicinities of particles of the first subset of particles in the first frame of image data, the particle filter processor being adapted, as the set of color likelihoods meets a prescribed criterion, to estimate a first region of the tracking object in the first frame of image data in accordance with the set of color likelihoods, and as the set of color likelihoods fails to meet the prescribed criterion, to apply a second subset of the set of particles to one or more frames of image data in the time series of frames of image data, for a second set of measurements to determine a set of luminance likelihoods based on luminance differences between frames of image data in the time series of frames of image data, and estimate a second region of the tracking object in a second frame of image data in the time series of frames of image data in accordance with the set of luminance likelihoods, the particle filter processor being configured to update the target color by a color in the first region or the second region, whichever is estimated.