Simple categories matching darker:

• wood,
• fruit,
• color,
• colors,
• shade,
• material,
• spirits,
• drink,
• stone,
• alcohol,
• paint,
• tint,
• hardwood,
• green,
• liquor,
• berry,

Members with property darker:

• then_red,
• romaine,
• cherry,
• mahogany,
• escarole,
• burgundy,
• sandstone,
• walnut,
• sapphire,
• berry,
• apricot,
• jet,
• whiskey,
• olive,
• teak,
• purple,
• agate,
• brandy,
• tan,
• strawberry,
• grey,
• basalt_sand,
• brazilian_walnut,
• red,
• dark_brown,
• bourbon,
• black_onyx,
• oak,
• aged_oak,
• cedar,
• light_pastel,
• dark_green,
• blood,
• blackberry,
• plum,
• royal_blue,
• rum,
• applewood,
• fig,
• raisin,
• denim,
• blue_colors,
• cognac,
• matrix,
• blue_sapphire,
• white_oak,
• pine,
• blue,
• mango,
• great_green,
• beech,
• bloodstone,
• green,
• black_currant,
• obsidian,
• dark_oak,
• organic_ink,
• ebony,
• gray,
• dark_cherry,
• ruby,
• rosewood,
• digestion_product,
• stained_oak,
• dark_grey,
• marble,
• emerald,
• melanin,
• navy,
• elderberry,
• black_cherry,
• raspberry,
• hematite,
• blueberry,
• light_mocha,
• deep_green,
• sycamore,
• orange,
• melon,
• brown,
• smoky_quartz,
• redwood,
• boysenberry,
• granite,
• prune,
• scotch,
• forest_green,
• golden_brown,
• onyx,
• emerald_green,
• opaque_paint,
• black_walnut,
• spinach,
• black_granite,
• tequila,
• maple,