Keeping that in mind, I'll try to explain my thought process here
Alchemy often starts with a raw, basic state that undergoes a series of processes. Refinement, and purification. And in philosophical terms, enlightenment. Our player character Egg is a literal and metaphorical "prima materia". This raw state I mentioned. As our Egg moves through the world, dodging shadow snakes (A not-so-subtle nod at the Jungian concept of confronting one's shadow, the journey through darkness, reflecting the process of acknowledging and integrating these repressed, hidden aspects of the psyche) and delving deeper into the shadows, the world gradually gets darker. From the lush, flower filled grass lands to a barren desert, to a weird, corrupted purple zone, to complete abyss, devoid of any light (And is this not a definition of shadows and darkness? An absence of light?). The "voice" that keeps giving you pats on the back for being such a good egg, even directly points at this shift and asks "Is this the source of all this darkness?". This is a rather heavy handed (or so I thought) depiction "nigredo", or blackening, where one has to confront and integrate the shadowy, chaotic aspects. Crucial for eventual transformation and metamorphosis, reflecting the journey through dissolution to eventual purification. (SPOILERS) The final transformation into a sun represents both "albedo" (whitening) and rubedo (reddening), stages in alchemy where purification leads to illumination and completion. The GOLDEN sun, (again, rather heavy handed I thought) signifies achieving the philosopher's stone. Or the ultimate metamorphosis / enlightenment / purity. (SPOILERS) Light represents the ultimate goal of illumination and wisdom, achieved through a transformative process. Darkness represents ignorance, and challenges one must face to achieve this enlightenment. In mystical, philosophical and alchemical traditions, becoming sunlike / goldlike symbolizes the culmination of an alchemical journey.
But yeah, I get it. It was probably way too vague overall.