Why are dog dicks red?

I went to a friends recently and her dog was humping EVERYTHING. Disgustingly this unfortunately included my leg, which resulted in me showering at a friends for the first time.

Do to this constant humping I got several views of his dick that I did not ask for. But I did notice that it was weirdly like, red? With a lot of purple too. It was gross, like some weird alien thing.

Why is it this reddish/purple color?