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.