Cancer cell biologist here. Cancer cells, as others have mentioned, are just cells that are dividing uncontrollably. Some cancer cells will be pigmented, if they're derived from cells that produce pigments, but most are clear and have no discernible color.

For example, various types of skin cancer will be pigmented because skin can produce melanin. In this case, they'll look brown, tan, or black. But, something like bone cancer will be clear under the microscope.

While you can detect cancer cells based on their shape and the shapes of their innards, this typically requires various stains, eg, a stain that specifically colors DNA so you can see the shape of the nucleus.

If you or your son have more questions, feel free to DM me!