In a 2-qt. saucepan, combine rhubarb, 1 qt. of water and sugar; bring to a boil. Reduce heat; cover and simmer for 20 or until rhubarb is tender. Cool for 15 minutes.
Place in a blender; process until smooth. Return to pan. Dissolve cornstarch in remaining water; stir into rhubarb mixture. Bring to a boil; boil for 2 minutes; stirring constantly. Add food coloring if desired; mix well.
Add a small amount of soup to egg yolk, stirring constantly; return to pan. Chill. Just before serving, beat cream until very soft peaks form; fold into soup.