The means of expression is the optical mixture of tones, of tints (of local color and the illuminating color: sun, oil lamp, gas, etc.), that is, of … - Georges Seurat

Gaiety of tone is the luminous dominant, of tint, the warm dominant, of line, lines above the horizontal
Calmness of tone is the equality of dark and light; of tint, of warm and cool, and the horizontal for line.
Sadness of tone is the dark dominant; of tint, the cool dominant, and of line, downward directions.

Aesthetic:
Art is Harmony.
Harmony is the analogy of opposites, the analogy of similarities of tone, of tint, of line taking account of a dominant and under the influence of the lighting, in combinations that are gay calm or sad.

Opposites are:
for tone, a more luminous/lighter one for a darker one.
for tint, the complementaries, that is, a certain red opposed to its complementary, etc.
Red — Green
Orange — Blue
Yellow — Violet
for line, those making a right angle.