Steven and Ed are right. There is a lot of a particular protein in our red blood cells that is called hemoglobin, and each of these hemoglobin molecules contain 4 sub-proteins. Each of these sub-proteins has one iron II atom sitting in its center. The way that this iron is arranged it absorbs other colours and only leaves red behind, which we then see. The default colour of blood, which is yellow, simply gets “swamped out” with all the hemoglobin-containing blood cells!