<p>In color particle tracking velocimetry (PTV), the number of colors of illumination light has been increased to improve the depth accuracy. However, structural problems in the optical system cause low depth accuracy in identifying the colors of particle images. If the number of colors is increased arbitrarily, colors close to those in the adjacent layer will be placed, and the depth accuracy will not improve. We developed a color PTV algorithm that achieves high depth accuracy even with a small number of colors by arranging layers with discontinuous color changes and tracking the color change of the image as the particle passes through each layer. In this paper, we describe the new color PTV and then use it to investigate the behavior of solid particles in a gas–solid two-phase pipe flow. We observe that particle behavior changes depending on the presence or absence of dunes inside the pipe and that particles are transported long distances inside the pipe by spontaneously generated dunes.</p> Graphical abstract <p></p>