Combine warm water, yeast, and 2 teaspoons sugar in a small bowl. Mix well; set in a warm place about 10 minutes.
Combine 4 cups flour, 1/2 cup sugar, salt, nutmeg, and lemon rind; add warm milk, melted butter, egg yolks, and yeast mixture. Beat until smooth.
Turn dough out onto a lightly floured surface; knead in enough remaining flour until dough is no longer sticky. Continue kneading about 10 minutes until dough is smooth and elastic.
Place dough in a well-greased bowl, turning to grease top. Cover and let rise in a warm place (85°), free from drafts, about 1 1/2 hours or until doubled in bulk.
Punch dough down, and place on a lightly floured surface. Sprinkle with citron, if desired, kneading until citron is evenly distributed. Shape dough into a cylinder 30 inches long.
Place cylinder on a buttered baking sheet; shape into a ring, pinching ends together to seal. Place a well-greased 2-pound coffee can or shortening can in center of ring to maintain shape during baking.
Press pecan half gently into ring from bottom so it is completely hidden by dough. Cover and let rise in a warm place about 45 minutes or until doubled in bulk.
Bake at 350° for 30 minutes or until golden brown. Remove coffee can immediately. Allow cake to cool. Drizzle cake with glaze; sprinkle with sugar crystals, alternating colors.