Either way, you mix tomato juice with apple juice. Proportion should make it clearly thinner than tomato juice, without losing tomato flavour. That's it.
Reason for proposed names:
- ketchup as such contains tomato paste and apple sauce. So, a drink with tomato juice and apple juice would be a drink version of it - or a liquid version, hence "ketchup drink" or "liquid ketchup" - except it doesn't sound very good;
- while some apples are red, tomatoes are generally red, so tomato mixed with apple would in that sense be "red apple" as in "red plus apple" - but while red apple is very much better sounding, it is also less clear ... so, what about "red plus apple"?
Or do you have other suggestions? Comment box is in fact open, here as on other blogs./HGL