Firstly, congrats on your new boa
Boa Contrictor Imperator:
These include Colombians, Hog Isle Boas, Central Americans, Mexicans. Often labelled as redtails, these are not true redtails, and the name came about as a marketting ploy a few decades ago. They usually finish around 5-8 feet on average (however they have been known to <I>occassionally</I> reach lengths of 10' - this is quite rare). Some of the differences include scale counts, saddle counts (they typically have 22 or more), saddle shape (BCC= <> BCI= )( ), shorter heads, and less colourful. The only surefire method is a saddle count, as colour and shape is not always accurate, as some Colombians can rival BCC in those categories, and saddle counts can be a good measure, but as I understand it some people are now selectively breeding for fewer saddles in BCI. They also are less demanding in husbandry requirements and a better first boa. These are typically what you find labelled as redtails in pet stores (unless they are reptile specialty stores).
Boa Contrictor Constrictor:
These include Guyanese, Surinames, Peruvians, and (rare) Colombians. These attain lengths of 6-11 feet, 11 feet considered very large and is not commonly found. In general, they have fewer saddles (14-22) which are usually peaked in the middle, are more colourful, have more elongated heads, and different scale counts than BCI. They are reputed to be more sensitive temperament-wise than BCI, which I find to be quite true. They are more sensitive husbandry-wise as well, and are slower to mature than BCI.