The whole idea behind painting the back side of the siding is to prevent the wooden siding from absorbing frost melt in the spring time.
Modern houses are built a lot more air tight than older homes, but even in modern houses there are places where warm moist air escapes into the exterior walls and forms frost. Come spring time, that frost melts and that melt water would otherwise be absorbed by the bare wood on the back of the siding. As that moisture wants to evaporate to the outside, it causes the paint to peel on the exterior painted surface of the siding.
Painting the back side of the siding with a latex paint allows the melt water trapped in the wall to evaporate through the painted wood, but doesn't allow the wood to absorb liquid water. (H2O molecules pass relatively easily through latex paint, but not water. That's because the average distance between H2O molecules in liquid water is much larger than the spaces between the plastic paint molecules.)
If it were me, I would buy any INTERIOR or EXTERIOR latex paint that's been mistinted to the wrong colour for $10 per gallon.
Exterior latex paint is essentially interior latex paint to which mildewcides and UV blockers have been added. On the back side of the siding, you're not going to need UV blockers in the paint because it'll never see daylight. And, you really don't need the mildewcides either because it's only going to be in early spring that there's water inside that wall. Mildew needs WATER to grow, not frost. And, for most of the spring, summer and fall, the interior of the wall will be dry enough that mildew wouldn't grow in there.
So, I'd use a interior or exterior mistint, but I'd buy the exterior mistint before the interior mistint just to be on the super safe side.