In color specification systems, three attributes describe a specific color: hue, saturation, and which third attribute?

Introduction / Context:
Describing color numerically or conceptually often uses three components. In the HSV (hue–saturation–value) and HSB (hue–saturation–brightness) models, the third axis complements hue (the dominant wavelength family) and saturation (color purity) by indicating overall lightness/brightness of the color.

Given Data / Assumptions:

• We are referring to common graphics/UI and imaging models.
• Terminology: 'value' is frequently synonymous with 'brightness' in HSB.
• Alternatives such as HSL use 'lightness' for a similar concept.

Concept / Approach:
Hue locates the color on the color wheel. Saturation measures how intense or diluted the hue is relative to gray. Value (brightness) measures how light or dark the color appears, independent of chroma. Together, these three let designers specify colors intuitively and adjust them predictably.

Step-by-Step Solution:
1) Identify the model named in the prompt: hue and saturation are given. 2) Recall common triples: HSV/HSB use 'value/brightness' as the third axis. 3) Exclude unrelated terms (metamer, grayscale, shading) that are not axes in HSV. 4) Choose 'Value'.

Verification / Alternative check:
Graphics software palettes often expose HSV sliders; the third slider is labeled value/brightness, confirming the standard nomenclature.

Why Other Options Are Wrong:
Option A: Metamer describes different spectra that look the same, not a color axis.
Option C: Grayscale is a set of achromatic values, not an attribute of chromatic colors.
Option D: Shading is a rendering effect, not a model axis.
Option E: Not applicable since 'value' is correct.

Common Pitfalls:
Confusing HSL and HSV; while similar, their third components (lightness vs value) behave differently near extremes.

Final Answer:
Value