In a large bowl, combine the rhubarb, sugar, cherries, tapioca and food coloring if desired; stir gently and let stand for 15 minutes.
Meanwhile, cut pastry sheet in half. On a lightly floured surface, roll out one half to fit a 7-in. pie plate. Transfer pastry to pie plate; add filling. Roll out remaining pastry to fit top of pie. Place over filling. Trim, seal and flute edges. Cut slits in pastry.
Bake at 400° for 40-45 minutes or until crust is golden and filling is bubbly. Cover edges with foil during the last 15 minutes to prevent overbrowning if necessary. Cool on a wire rack.