When the segmentation results are obtained, how to mark the different segmented areas with different colors, like the fish in the picture, how this is probably achieved
The most common solution is to create surface meshes from the labelmap volume and render these surfaces. The picture above appears to be rendered using this method.
If the segmentation is noisy or not very accurate or you want to preserve the original opacity of the volume then you may create an RGBA volume and render that using raycasting. You set the RGB channels from the labelmap and use the input volume as alpha channel. See example below and more details here.