Short answers, pulled from the story.
Chromaticism is a compositional technique that intersperses primary diatonic pitches with other pitches of the chromatic scale. Diatonic music uses only seven different notes from the twelve available notes on a standard piano keyboard, while chromatic music utilizes more than those seven notes.
Chromaticism began to develop in the late Renaissance period during the 1550s. Composers like Cipriano de Rore utilized these techniques often as part of musica reservata, Orlando Lasso contributed through his work Prophetiae Sibyllarum, and Nicola Vicentino explored these concepts in theoretical writings.
Richard Wagner expanded tonal harmony during the last half of the nineteenth century. His opera Tristan und Isolde exemplified increased chromatic usage, leading to rapid key shifts that caused tonality itself to begin breaking down by 1900.
Augmented sixth chords appear frequently in harmonic analysis and resolve to the dominant chord marked V. A specific augmented sixth chord contains notes such as A, C flat, D, or E flat, F, and resolves directly to the V chord.
From the late 16th century onward, chromaticism came to symbolize intense emotional expression where diatonic elements represented light and affirmation while chromatic elements symbolized darkness doubt and grief. This imagery has hardly changed for three centuries despite Stravinsky's skepticism regarding the association.