Cook plums with brown sugar, lemon juice, and just enough water to prevent scorching.
Cook in tightly covered pan over low heat until plums are very soft; let cool.
Beat egg whites with dash of salt and cream of tartar.
Add food coloring and sugar gradually, beating until peaks form.
Beat in cooked plums, small amount at a time, to keep mixture light and fluffy; use rotary beater.
Chill in sherbert glasses at least 3 hours before serving.