Works of art that express a happy/joyous tone might depict characters that are in cheerful states and loving in their interaction with others. Beyond representation, stylistic elements such as bright colors, light and medium values, and a glowing/radiating light might evoke a tone of joy and happiness.
An artist might impart a happy/joyous tone to a piece due to his or her own emotional state, or the artist might choose to evoke this tone as a reflection of the characters being portrayed. A happy/joyous tone might also be adopted to suit a piece meant for a setting or purpose that demands an expression of cheerfulness.
Featured: The Wedding Dance by Pieter Bruegel the Elder.