For integrals, we would say that "b and a are the limits of integration".
The notation "lim x->0 1/x" would be read as "the limit of 1 over x as x goes to zero." In general, "lim" is short for "limit" of whatever follows it, with respect to what is below the "lim" symbol. Rarely, I have also seen the notation "l.i.m." used for the limit in mean, i.e. the limit with respect to the L^2 norm.
But really, I'm team Green. b goes more or less in the place Red shows it (or maybe halfway between where Red and Blue show it), but a goes to the left of the integration symbol, mirroring where the b goes relative to the curve at the end of the ∫
Best answer, although I work with delta "functions" a lot so I actually have to be careful picking which interval with boundary {a,b} to pick (for example, if I integrated δ(t-a)+δ(t-b) over all t in (a,b), I'd get 0, but if I integrated those deltas over (a,b] I'd get 1, and integrating over [a,b] would give 2).
Also I do have to do integrals with parameters and multiple variables so I can't really leave out the differential.