Short answers, pulled from the story.
Chromaticism is a compositional technique that mixes primary diatonic pitches and chords with other pitches of the chromatic scale. Diatonic music uses only seven notes per octave, while a standard piano keyboard offers twelve, and chromaticism draws on the extra five.
Chromaticism began developing in the late Renaissance, notably in the 1550s. It appeared in the music of Cipriano de Rore, in Orlando Lasso's Prophetiae Sibyllarum, and in the theoretical work of Nicola Vicentino.
Richard Wagner used chromaticism extensively, especially in the opera Tristan und Isolde. His increased use of it is cited as one of the main signs of tonality's breakdown in the nineteenth century.
Chromatic chords catalogued by theorists Shir-Cliff, Jay, and Rauscher in 1965 include dominant seventh chords of subsidiary keys used for modulation, augmented sixth chords, Neapolitan sixth chords acting as chromatic subdominants, diminished seventh chords acting as a chromatic vii7, altered chords, and expanded chords.
A line cliche is a chromatic line that moves against a chord that otherwise stays still. The major line cliche moves from the fifth of a chord to the sixth and back, forming a I, I+, vi, I+ progression from the tonic, while the minor line cliche descends from the root through the major seventh to the minor seventh.
Chromaticism has symbolized intense emotional expression since the late 16th century, when a repeated melodic semitone became linked to weeping through the lament bass and pianto. Pierre Boulez wrote in 1986 that the chromatic has long symbolized darkness, doubt, and grief against the diatonic's light, affirmation and joy.