In a medium saucepan, sprinkle gelatin over 1/2 cup of the cold water; stir over low heat until gelatin is dissolved; add sugar, salt, remaining 1 1/2 cups water and lemon juice; stir until sugar is dissolved.
Remove from heat, add salad dressing and food coloring (if using), stir until well blended; stir in cabbage, carrots and celery.
Cut part of the green pepper into rings and reserve for later use.
Chop remaining green pepper and stir into salad.
Pour into an oiled 6-cup mold; refrigerate until firm, about 4 hours.
To serve, unmold onto lettuce leaves and garnish with green pepper rings.