It should be noted that although Animus, Tower Corp, and Jotunheim are listed as acts 2-4, they may be played through in any order.
Act 1 - Homeworld
Homeworld is the first act of the game. This act takes place in Lore's House, the surrounding forest, The Spine, the Trial Between Worlds, and ends in the Dark World. The color palette tends to consist of cool, desaturated, and earthen tones.
Act 2 - The Tower
The Tower is one of three acts available after Homeworld. It takes place in what appears to be a deteriorating office building in an empty void. The color palette solely consists of red, black, white, and very rarely gold.
Act 3 - Animus
Animus is one of three acts available after Homeworld. This act takes place in a village of anthropomorphic animals, with surrounding areas including a desert, mushroom-filled cave, and pond. Animus' color palette is generally autumnal, though some areas have cooler, watery colors.
Act 4 - Jotunheim
Jotunheim, being either Muspelheim or Niflheim (based on whether you turn the stove on or not), is one of the three acts available after Homeworld. The worlds appear to take place in the kingdom of their respective ruler, each containing large amounts of industrial machinery and pipework. Muspelheim's color palette tends to consist of warmer, fiery colors, while Niflheim's palette tends to be icy and metallic.
Act 5 - ???
As of writing this, all that is known about Heartbound's fifth act is that it will consist of nine maps. This is the same number of maps as in Homeworld, though it is unknown if this holds any significance.