There actually are some ways you can tell cancer cells from non-cancerous cells using color/appearance! Cancer cells generally have a higher nuclear/cytoplasmic ratio than non-cancerous cells. In other words, in cancer cells the cell nuclei are larger compared to the size of cell they’re in. This can be seen under a microscope, but the difference between nucleus and cytoplasm is hard to see with its ‘natural’ colors. But you can see the difference if you use dyes which color the nucleus one color and the cytoplasm another.

The cell also has various optical properties which are subtle or otherwise not visible to the naked eye which can be used to visually distinguish cancerous and normal cells. For example, human tissues is fluorescent. If you shine light at a specific wavelength (often in the UV) the tissue lights up (fluoresces) at a different wavelength (color). Cancer cells have different fluorescent properties than normal cells, and this can be seen using a specialized microscope.

You can also apply dyes to make cells and their structures more easily visible. There are certain dyes which will stick to cancer cells but not non-cancer cells. These can be used to make it relatively easy to spot cancerous tissue. However this requires a dye to be applied which can be impractical depending on how toxic the dye is, where the site is in the body (inside of the mouth is way easier to get a microscope and dye in than inside the heart for example)