There's enough stuff to stop it from being totally clear, but the red color is from hemoglobin. Modified hemoglobin can produce other colors, like the chocolate-brown blood seen in methemoglobinemia, or the bright red of carboxyhemoglobin from carbon monoxide poisoning. read more
The iron in the hemoglobin is what gives the color of red or dark purple depending on the O2 content. Without that it is just the blood plasma. read more