Colormap turns numeric pixel values into colors/brightness on your screen.

A colormap has configurable contrast limits that determine the pixel value range that is rendered linearly to display the image.

Colormap settings must be responsibly chosen to convey the intended scientific message and not to hide relevant information.

A gray scale colormap is usually preferable; especially blue and red colormaps are not well visible for many people.

For high dynamic range images multicolor colormaps may be useful to visualize a wider range of pixel values.

A watershed transform can separate touching objects if there are intensity valleys (or ridges) between touching objects. In case of intensity ridges the image needs to be inverted before being subjected to the watershed transform.

To separate object by their shape, use a distance transform on the binary image and inject this into the watershed transform. It is often good to smooth the distance transform to remove spurious minima, which could serve as wrong seed points and thus lead to an over-segmentation.