We have red blood because it contains iron in the haemoglobin. If you use a different metal, for example copper, in a hemocyanin, it can do the same thing as haemoglobin – bind oxygen, but it would be a different colour, blue. Creatures like lobsters have blue blood because of this.
The colour of blood really appears as either a bright bright red colour or a dark wine colour. The reason for the colour change is based on how much oxygen is within the blood sample. A blood sample that has lots of oxgen is really bright red whereas a blood sample with low levels of oxygen is dark red in appearance.