Single (human) cells wouldn't appear as anything to the naked eye. Unless your naked eye is a microscope, which would be dope.

You've gotten lots of good comments describing what tumors look like, but if you're wondering what color the actual cells are, the answer is that they are usually off-white (just like most cells in the body, which cancer cells derive from). The colors others are describing in solid tumors or organs are usually due to the presence of different types of cells (like red blood cells) or certain fluids (like the yellow of bile). If you were to remove the tumor, squish it up and take out just the cancerous cells, to the naked eye they would most likely appear off-white.