"My apples are not red."
Translation:Mis manzanas no son rojas.
A temporal problem I think. If you have Granny Smith apples they will never be red and ser makes sense. If you have Macintosh apples which haven't turned red yet then I'd guess that estar makes more sense.
Mis is when the object (in this case, apples) is plural.
Mi is for singular objects (apple).
- The object in this case is plural. The color reflects this too, hence "rojaS"
- The object is feminine, hence the color is too. Therefore "roja" and not "rojo", which is masculine.