The regions of Hollow Knight all share a similar theme, they have a distinct color to them, whether it be the green of green path to the blue of the city of tears, each of these use color theory to help invoke a certain feel to each of the regions.
Dirtmouth
Hollow Knights Dirtmouth is characteristically black, as it is known as “The Fading Town,” where it starts out almost completely empty. Black typically is used with isolation, which in this case, is very prevalent at the start of the game, throughout the game, you rescue npc’s and as you do, each shop door opens bringing more and more life into the town. This removes some of the black from the town, which makes the town seem less empty.
White Palace
Hollow Knights White Palace mainly uses white as its color. White Palace was where the ruler of Hollownest lived, and it is reflected with its colors. The ruler (also known as The Pale King) was what is known as a Pale Being, and Pale Beings are characteristically pure. Because of this, The Pale King made his palace to be white, which is known as a very pure color.
Greenpath
Greenpath is, well, green, and that is shown in its gameplay as it can bring a very calm feeling and it lowers stress. It is one of the earlier areas of the game, and is very easy to get through. The enemies are very slow and easy to kill, and the platforming is very simple. Additionally, Greenpath has a lot of nature in it, which green brings out a lot of that feeling.
City of Tears
Conversely, the city of tears is blue, which brings about feelings of sadness. The city is the center of Hollow Knight and it used to be full of life. However, as the player arrives, the city is falling apart. The few residents of the city are all mindless, causing them to attack the player. Additionally, the city has rain constantly, which adds to the atmosphere of the city.