Classification (spectral vs non-spectral colours): Three are spectral colours in the visible spectrum; one is a non-spectral composite colour. Identify the odd one out.

Introduction / Context:
Colour classification questions often distinguish spectral (component of a simple rainbow spectrum) from non-spectral colours (which do not appear as a single wavelength band). Green, yellow, and violet are spectral colours within the visible range. Brown is non-spectral; it is perceived contextually (typically as a low-luminance, desaturated range near orange). Thus, brown stands apart from the others in basic colour science terms.

Given Data / Assumptions:

• Spectral colours arise from single-wavelength light components in the visible spectrum.
• Green, yellow, violet → spectral.
• Brown → requires complex viewing context/mixture; not a single spectral band.

Concept / Approach:
Partition the set by spectral status. The unique non-spectral colour is the outlier, enabling a crisp 3-to-1 grouping without relying on cultural naming or subjective shade boundaries.

Step-by-Step Solution:

Verification / Alternative check:
Visualize a prism or rainbow: green, yellow, and violet visibly occur; brown does not appear as a band.

Why Other Options Are Wrong:

Common Pitfalls:
Equating “common” colour names with spectral status. Not all everyday colours are spectral; brown and pink are classic non-spectral examples.

Final Answer:
Brown