Dissolve yeast and sugar in 1/2 cup of the warm water. Let stand until bubbly. Combine saffron and remaining 1 cup water; add to yeast. Stir in 2-1/2 cups of the flour, 2 whole eggs, 2 egg whites, margarine, and salt. Beat with an electric mixer on low speed for 30 seconds; beat on high speed 3 minutes. Stir in as much of the remaining flour as you can. Turn dough out onto lightly floured surface. Knead in enough of the remaining flour to make moderately soft dough (3 to 5 minutes). Place in greased bowl. Cover; let rise in a warm place until nearly double (about 1 hour). Punch dough down. Cover; let rest 10 minutes.

Divide dough in half. Shape half of the dough into three 18-inch-long ropes. On a greased baking sheet, loosely braid (begin in center and work toward ends). Moisten and seal ends.

Set aside one-third of remaining dough. Shape the two-thirds portion of dough into three 14-inch-long ropes. Braid and seal ends. Moisten top of 18-inch braid. Place 14-inch braid on top. Shape remaining one-third portion of dough into three 12-inch-long ropes. Braid and seal ends. Moisten top of 14-inch braid. Place 12-inch braid on top. Cover loosely; let rise until double (about 30 minutes).

Combine egg yolk and 1 tablespoon water. Brush on loaf; sprinkle with seeds. Bake in a 375 degree F oven 35 to 40 minutes or until bread sounds hollow when lightly tapped. Cover loosely with foil the last 10 minutes, if necessary, to prevent overbrowning. Cool on rack. Makes 1 loaf of challah (20 servings).

Bake and cool bread. Wrap in foil and place in a freezer container or large freezer bag; seal, label and freeze up to 1 month. Thaw 1 hour at room temperature before slicing.