 used for each arm of the palette, (b) chroma and luminance trajectory
are balanced between the two arms, (c) the neutral central value has zero chroma.
To specify such a palette a vector of two hues <code>h</code> (or equivalently <code>h1</code>
and <code>h2</code>), either a single chroma value <code>c</code> (or <code>c1</code>) or a vector
of two chroma values <code>c</code> (or <code>c1</code> and <code>cmax</code>), a vector of two
luminances <code>l</code> (or <code>l1</code> and <code>l2</code>), a