The types, monster IDs and so on are colored red because they're constants. You can add them in the init_constants script. This is where the type constants are defined, for instance:
(You can define constants anywhere in GMS2 but it felt easier to have them all in one place for easy access)
Keep two things in mind: each constant of each enumeration needs to have an unique value (e.g. type_DRAGON should have value 8), and you should update the MAX constant to be one higher than the highest constant (e.g. after adding the dragon type, TYPE_MAX should now be 9) so the loops that goes through some things won't skip the newly added data.