Doing the blood vessels on this thing is fun. The preserved anatomical specimens I'm using for inspiration were made by injecting coloured wax into the specimen's blood vessels to make them keep their shape. The wax also had antibacterial properties, due to the fact that the colouring agents were toxic, and so helped to preserve the specimen. Corrosion casting is an important learning tool for medical students because it allows you to see what the blood vessels look like in three dimensions. Today the technique has come a long way from the early coloured wax casts, and you can see some excellent modern examples here.
What I've done here may seem like a far cry from elementary school paper mache balloons, but these blood vessels are made using only paper mache. So are the trachea and esophagus you can see in the photos, and in fact everything else except the teeth.
How do the veins get to be that colour? Payne's grey and cadmium red with a coat of polyurethane to give them a bit of a sheen. Works every time.