Halloween’s almost here, which, for a large number of costumes, will require a liberal dousing of fake blood to complete the look. You probably already have a pretty good idea of the reasons behind the red colouration of human blood that fake blood mimics. However, red is not the only blood colour available – it also comes in blue, green, violet, and even colourless varieties – and this is a result of the specific chemicals that make up blood in different organisms.