In the Pokemon Blue codebase they use 2 flag variables to distinguish between the different games, these are `pokemon_type` and `pokemon_type_blue`. The name of the second flag definitely hints that it was added later in development.

These are the values for each game:

    Green:  pokemon_type=0 pokemon_type_blue=0
    Red:    pokemon_type=1 pokemon_type_blue=0
    Blue:   pokemon_type=1 pokemon_type_blue=1

The general pattern for branching between game variations looks like this across the codebase:

    ifn pokemon_type
      ifn pokemon_type_blue
        ; blue
      else
        ; red
      endif
    else
      ; green
    endif

Conditional assembly directives like `ifn` are resolved statically during assembly, so only the code between matching conditions is included as part of the output. To anyone interested in exploring this a bit more, I'd recommend reading Chapter 8 Section 13 of the DOS version of The Art of Assembly Language Programming [0], which starts on page 43 of the linked PDF.

Bonus fun-fact: In the Pokemon Yellow codebase it says `pokemon_type=1` is yellow, while `pokemon_type=0` is pink! This suggests to me that the idea of Pokemon Pink with Jigglypuff as your starter was probably being floated around but it was eventually scrapped. (The only remaining options for a pink starter pokemon with a pink evolution in the original 151 would be Clefairy and Slowpoke, neither of which are very cute.) The idea of Jigglypuff as a starter is further supported by her appearance alongside Pikachu on the roster of the original Super Smash Bros. which seems rather unexpected unless they had bigger plans for her.

[0] http://www.plantation-productions.com/Webster/www.artofasm.c...