The content categories that are generally used are published as part of the DVB standard. The DVB standard is put out by the European Telecommunications Standards Institute (ETSI – etsi.org). The portion that deals with content categories is EN 300 468 (ETSI European Standard (EN) 300 468, current version 1.15.1 released March 2016): "Digital Video Broadcasting(DVB); Specification for Service Information (SI) in DVB systems" (http://www.etsi.org/deliver/etsi_en/300400_300499/300468/01.15.01_60/en_300468v011501p.pdf).
From pages 47–48, here are the defined categories:
Movie/Drama:
* Movie/Drama (general)
* Detective/Thriller
* Adventure/Western/War
* Science fiction/Fantasy/Horror
* Comedy
* Soap/Melodrama/Folkloric
* Romance
* Serious/Clasical/Religious/Historical movie/drama
* Adult movie/drama
News/Current affairs:
* News/Current affairs (general)
* News/Weather report
* News magazine
* Documentary
* Discussion/Interview/Debate
Show/Game show:
* Show/Game show (general)
* Game show/Quiz/Contest
* Variety show
* Talk show
Sports:
* Sports (general)
* Special events (Olympic Games, World Cup, etc.)
* Sports Magazines
* Football/Soccer
* Tennis/Squash
* Team sports (excluding football)
* Athletics
* Motor sport
* Water sport
* Winter sports
* Equestrian
* Martial sports
Children's/Youth programmes:
* Children's/Youth programmes (general)
* Pre-school children's programmes
* Entertainment programmes for 6–14
* Entertainment programmes for 10–16
* Informational/Educational/School programmes
* Cartoons/Puppets
Music/Ballet/Dance:
* Music/Ballet/Dance (general)
* Rock/Pop
* Serious music/Classical music
* Folk/Traditional music
* Jazz
* Musical/Opera
* Ballet
Arts/Culture (without music):
* Arts/Culture (without music, general)
* Performing arts
* Fine arts
* Religion
* Popular culture/Traditional arts
* Literature
* Film/Cinema
* Experimental film/video
* Broadcasting/Press
* New media
* Arts/culture magazines
* Fashion
Social/Political issues/Economics:
* Social/Political issues/Economics (general)
* Magazines/Reports/Documentary
* Economics/Social advisory
* Remarkable people
Education/Science/Factual topics:
* Education/Science/Factual topics (general)
* Nature/Animals/Environment
* Technology/Natural sciences
* Medicine/Physiology/Psychology
* Foreign countries/Expeditions
* Social/spiritual sciences
* Further education
* Languages
Leisure hobbies:
* Leisure hobbies (general)
* Tourism/Travel
* Handicraft
* Motoring
* Fitness and health
* Cooking
* Advertisement/Shopping
* Gardening
Personally, I'm not fully certain how Kodi decides which color overlay matches which genre. In the default skin of Estuary, these images are are stored in @skin.estuary/media/windows/pvr/epg-genres/@, however they have unhelpful filenames of @0.png@, @16.png@, @32.png@, etc. One could almost believe that the number equates to the first bit of "content nibble" defined for the SI, but I don't believe that the colors displayed match the categories if you do that. (Also, there are more images in the Estuary resources than there are categories defined.)
In any case, perhaps this will give you some direction of what category names are acceptable/expected when parsing guide information. Although the US does not use the DVB standard, most software seems to be based upon DVB standards rather than the US/North America's ATSC/SCTE standards.