As I am late in my sixth decade I suspect that age is not the explanation. Perhaps regional variation, as I hail from the north, but the original question triggered a survey of friends from around the country who all said tomato or red sauce. Maybe it is related to England's great undiscussed and one of them is U and the other is not?! :)
Age and region are good guesses, but I believe part of the answer to the confusion here may be something nobody's mentioned yet: class. According to my Cockney boyfriend, calling ketchup red/tomato sauce is more a working class thing.
In my middle-class experience, red/tomato sauce is the Italian sauce you put on pasta. North Americans might call this marinara. To me, the 'red' in red sauce actually distinguishes it from a white sauce, such as a carbonara.
Sapiophia, Ah you're middle class, actually U or non U is totally class related, lavatory v toilet, napkin v serviette. Tomato sauce as the component of an Italian meal is far younger in English than than this particular divide and as for carbonara, I must have been at university before anything "Italian" beyond spag bol appeared on my plate - and we always said lavatory on pain of grand-mama's displeasure. So, I still hold for regional, Cockney is still the south and not the north where I was dragged up, although admittedly whilst wiping my mouth with a napkin. Apologies to non English participants, this is a rather parochial debate.