I want to share my approach to color in fantasy and science fiction illustration specifically, because speculative fiction gives you unusual latitude with color that realistic illustration doesn't have.

The temptation: using every color available because the setting is fictional and anything is possible. This produces visual chaos.

The better approach: establish the in-world logic of the color. What are the light sources in this world? What are the dominant materials and their surface colors? What is the atmosphere like? The in-world logic constrains color decisions in ways that produce coherence even in fantastical settings.

For bioluminescent environments: the light comes from the subjects themselves. Organic greens, blues, and purples are dominant. The ambient environment (cave walls, water) is largely dark, showing only what the bioluminescent light falls on.

For alien planet scenes: I establish a different primary atmosphere color (purple sky, orange sky, red-brown sky) and then work through the lighting logic consistently. The alien sky color modifies every shadow and every ambient light surface the same way our blue sky does on Earth.