I remember hearing that the tumor where HeLa cells originated from was purple and jelly like, and I've seen pictures from studies with rats and HeLa tumors, and their tumors appear to also be purple. So what makes these tumors purple?
HeLa cells themselves are not pigmented, and usually appear translucent/gray under light microscopy. The growth medium may however be colored, and a common color is magenta.
The tumors themselves will appear purple, as most tumors do, due to the broken vascularization. Tumors commonly secrete growth factors leading to angiogenesis, but the resulting vessel networks are convoluted and not well organized, commonly full of anastomoses. The resulting poor vascularization leads to significant hypoxia, which results in a bluish coloring of the blood.