"Do you have a light?"
Translation:Tu as du feu ?
That should be accepted, because it is used in writing of course, but also in oral (I do it) and that is the closest translation to the English form.
So "do you have a light" should accept : "as-tu" + "tu as" + "avez-vous" + "vous avez".
Why is "un feu" not accepted? It says "You used the indefinite "un" here, instead of the definite "du"."... well "a light" is indefinite, right?
Because if you need a lighter or matches, you will ask for "du feu" (partitive, since "feu" is not countable)
Is this colloquial, like for when smoker is asking another if happens to have light (matches, for instance), or you can use it to say you want to buy light fixture, etc?
It is not colloquial, but the standard way to ask for a lighter or matches, because "du feu" means "(some) fire".
So, of course, "du feu" does not work in a context where you want to buy a lamp or light fixture, etc.
"feu" is fire, with a flame and not much light.
"lumière" is light, produced by different ways, with or without flames.
I put "as-tu un feu. Wrong.