4 ideas
A colour on the canvas has no fixed value of its own. Its apparent hue and brightness are set by the tones placed around it, so a dull, muddy mixture can read as glowing flesh when surrounded by the right contrasts. The painter therefore works on the relations between tones, not on the correctness of any single patch.
Shadows and half-tones carry their own hue, often the complement of the lit surface, together with coloured reflections from nearby objects. Modelling should come from shifting hue and temperature, not from adding darkness.
The first sketch holds the whole conception and its energy. The painter must sacrifice details to the effect of the whole, and knowing what to leave out is the core of the craft, not a shortcut.
In an entry of April 1849, Delacroix records a carriage ride through Paris with the ailing Chopin, who explained that in music, logic, specifically counterpoint and the fugue, is the foundation of expression. On Chopin's account, science and inspiration are not opposed, and knowing the rules is what frees feeling.