Surprising(ly) Easy Ombre Cake

Last week at the Library we had a retirement party for Chris from the Children's Department.  She loves the color purple, and one of her coworkers, Jen, asked me to make her a cake, preferably purple.  When I dropped off the cake in the party room, I could see that Jen was disappointed:  the cake I had made was chocolate with multicolored sprinkles all over.  I wrote on it - "Chris, have a