Compare to "contentment" in this list--both connect to happiness, but "merriment" is more momentary and dependent on "activities that are enjoyable or amusing" while "contentment" is more lasting and dependent on one's ability to control one's attitude towards life (which can be taught).
Their merriment and the brilliant red color of their clothes and of Dragon himself seemed to warm the cold landscape.
"Foundation" and "base" are synonymous nouns here, but "base" refers to the bottom of the mountain, while "foundation" refers to the bottom of the earth, and the use of both words emphasizes how never-ending the mountain seems to Minli.
Minli could not see the tip or the bottom of it, as it seemed to grow out of a gorge so deep that the base must have been at the foundation of the earth.
Compare to "lustrous" in this list--although the adverb "softly" makes "luminous" seem dimmer than "lustrous" (defined with the brighter "brilliant"), the strength of the light described in both example sentences seems to be equal, especially since both are connected to the adjective "translucent" ("allowing light to pass through diffusely").
The shabby walls and worn stones seemed to shimmer as if a translucent silk veil covered them, muting any flaws and transforming the house into a dwelling of luminous light and delicate shadows.
"Flourish" is synonymous with "thrive" so using both verbs seems unnecessary, but both also mean "make steady progress" so the crops could thrive first, and then flourish. To give up an activity just when he sees progress, growth, and success emphasizes how discontented Wu Kang was with his life.
So when his crops thrived and flourished, he decided farming was not satisfying enough for him and the day he reaped his successful harvest, he told his friends that he was leaving the countryside to move to town.
The structure of the sentence emphasizes that the current experiences are opposite of the previous ones--the antonym of "bland" would be the usual "savory" flavor of the rice; the antonym of "colorful" is not usually "rough" but it is here because the two words produce opposite feelings in the wearer; and the antonym of "free" can be "stifle" ("smother or suppress").
The rice that filled their bowls began to taste bland, the clothes she had liked for their colors now felt rough, and the house that she had run freely around in had become stifling.