Genre and series . Redwall, Alex Rider and Diary of a Wimpy kid are at the top of my shelf. "Made my childhood" books ig. Manga and light novels on the second rack on my shelf. Other books are in another in another room arranged according to genre.
I try and keep series together, and I do put a small amount of effort into putting things with genre as well, but this isn't something I care too much about if they fit more conveniently in a different order.
Always alphabetically by author. I need to redo my shelf eventually cuz I have more nonfiction to shelf and I keep those separate from fiction; so alphabetically for both but not together. From top left to bottom right; nonfiction goes first, then fiction.
I don't really organize by genre, but I always keep series books together, in the proper series order. Other than that it's whatever fits on the shelf and looks best.
By author first, then series. Organizing by color may look nice, but it is complete madness if you've got books that are in series. Organizing by genre wouldn't work for me, since I only read 1 genre XD