Hi floyvera,
One possible explaination I'd like to offer for the bright arange colour is, when the Horntail / Hornet-whatever, is seen angrly buzzing around at high speed, it's sometimes difficult for the human eye to focus on it and the black and yellow colours seem to merge together thus giving the appearance of a stronger more orange colour, I've noticed this colour blurring with the hornets that feast on the fallen apples in my garden. so far I've not been stung and I find keeping a respectful distance is the best form of defence as they are non agressive if unless they feel theatened.
I've only seen a Horntail once, they are related to the sawflies and I agree they certainly look formidable but it's nice to know they are harmless. Looking at the picture Cetti has kindly posted, is enough to send the most laid-back of people running hell for leather! Regards. Tbird+