Your critique reveals a crucial conflation between structural emergence and perceptual categorization, a distinction that clarifies why "purple" (as a color category) lacks the causal efficacy you ascribe to it. If you gift me some of your valuable reading time, let's dissect this.

1. Two Types of Emergence

- Structural emergence (tables/chairs): Arises from physical interactions between components. A table's causal power (holding objects) derives from its atomic structure creating macroscopic rigidity. These properties are observer-independent. A laser would detect the table's structural integrity even with no humans present.

- Perceptual categorization (color): Emerges from evolved neurobiology + cultural reinforcement. The "purple" label applied to red onions is a compression algorithm for "reflects 400-450nm + 600-700nm with minimal 500-600nm". This categorization has no causal power beyond its role as an information tag.

2. The "Objective" Color Fallacy

Your intersubjective agreement about purple stems from:

- Shared cone cell biology: 94% of humans have L/M/S photopsins with peak sensitivities at ~560nm (red), ~530nm (green), ~420nm (blue)

- Cultural conditioning: Modern color lexicons standardized via Pantone systems and CIE charts

Yet this consensus doesn't make purple an emergent physical property.

Consider this.

The Himba tribe uses "zoozu" for dark colors (blue/purple/black) and doesn't distinguish purple as a category

Industrial paint manufacturers recognize 12,000+ color terms, far beyond basic spectral labels

Your "purple" onion would register as #6A1B9A in HEX, 17.3° hue in CIELAB, arbitrary numerical tags, not causal agents

3. Causal Efficacy Lies Elsewhere

The chain you described:

Photons → Retinal Activation → Neural Coding → Avoidance

Contains zero causal nodes requiring "purple" as an explanatory variable. Replace "purple" with "wavelength combo X" and the physics/neurology remains identical. Contrast with a table's causal power. Replace "table" with "carbon lattice configuration Y" and you lose the explanatory utility.

4. The Qualia Dodge

You're right to reject qualia-centric debates, but the alternative isn't reifying color categories. Instead, recognize that:

a) The onion's surface selectively reflects wavelengths

b) Your visual system detects this pattern

c) Your brain applies a culturally-learned label

d) The label activates memory associations

The causal oomph lives in the biochemical aversion pathways, not the color label. Change the label (call it "ploobalooba") while keeping wavelength data and aversion remains. Change the wavelengths while keeping the label, and behavior shifts.

5. The Real Emergent Culprit

What does have causal power here is pattern recognition heuristics. Your brain evolved to:

- Create color categories as survival shortcuts ("red" = blood/danger)

- Link these to outcomes via associative learning

These heuristics are genuine emergent properties with causal effects, but they're neural algorithms, not spectral properties. The purple label is their UI, not their codebase.

TL;DR

You're mistaking the map (color categories) for the territory (wavelength interactions). Tables derive causal power from structural emergence, "purple" derives consensus from neuro-cultural emergence. One explains why plates don't fall through surfaces, the other why we argue about onions at barbecues.