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The game is built on real-life genetic principles and the best scientific knowledge of horse color genetics, updated whenever new discoveries are published. There are some fantasy genes thrown in for fun, too. These include quite a few Ice variants, the Rainbow Riot cluster (Watercolor, Plaid, and Nacre), and the Nexus group (Nexus, Axiom in three colors, and Wrong Warp).

The game includes the latest updates on the various Agouti alleles (Bay) and most recently the Dun alleles have been upgraded to include non-dun 1, scientifically confirmed, especially in horse breeds from the Iberian peninsula (Andalusians and Lusitanos) last year.

The Genetics Guide gives more information, although it's not quite up to date for the game yet.
https://www.huntandjump.com/geneticsguide/

We have Mushroom, a dilution gene identified in British Shetland Ponies, which is a real gene, and Chinchilla which totally wipes out red pigment (pheomelanin), a new fantasy gene.

As for whether the game is worth it, I've been playing it since 2011, my daughter since 2010, and some players who began in 2006 (I think) when the game was first created are still around.