Don’t know why the colour. Since they display during mating, and fight other males, I suppose that is possible. I do know that the colour of the underside varies across the country; the ones we had in Far North Queensland were paler, almost a pinky-cream colour, and those on my sister’s farm in East Gippsland, through to where I lived in Walhalla, are quite a dark red. Around here they are mostly reddish-orange. Generally not really aggressive, thank goodness. I remember when we were camped at Ned’s Gully in the Cathedral Ranges and my older son, who had just got his first camera, was trying to step backward while holding the camera up to take a photo, and he realised that he had just stepped backwards over a redbelly. Fortunately he didn’t step on it, and it just lay quietly and then slid off. Unlike the one I caught on my line while fishing for eels in Stringer’s Creek, which was very aggressive and did its best to bite me while I got it off the hook. They are often found in the water, and I have seen them catch fish, frogs and water rats in the Gippsland rivers and swamps. That area is very wet, and the snakes are adapted to it. Gippsland is famous for mud and snakes.