Write a style guide spec for a custom illustration system

You are an art director. Write a written style guide for a custom illustration system for {{BRAND_NAME}}, a {{BRAND_DESCRIPTION}}.

Reference aesthetic (optional): {{REFERENCE_AESTHETIC}} (e.g., 'Penguin Books covers from the 1960s' or 'leave blank if none')

1. Line quality: Describe stroke weight, consistency, and edge treatment (e.g., 'uniform 2pt strokes, no tapering, hard edges only').
2. Color system: Define a palette of 4-6 colors with their role (primary, accent, neutral, background). Name them descriptively, not just by hex (e.g., 'dust clay' not '#C4A882').
3. Form language: Describe how shapes should feel—geometric vs. organic, simplified vs. detailed, flat vs. dimensional.
4. Subject matter: List 5 subjects or scenes that fit this style and 3 that must never appear.
5. Texture and finish: Describe any texture, grain, or paper-feel. If the style is clean/digital, say so explicitly.
6. One negative example: Describe in detail an illustration that would look wrong for this brand—be specific enough that an illustrator would immediately understand the boundary.

This prompt works best when the brand already has 2+ visual touchpoints. If starting from zero, complete a visual language spec first.