1Line bottom and sides of 5-quart slow cooker with single piece of cooking parchment paper; spray with cooking spray.
2Make cake batter as directed on box, adding food color with the water.
3Pour batter into slow cooker. Place folded, clean dish towel under cover of cooker. This will prevent condensation from dripping down onto cake. Cook on High heat setting 45 minutes. Carefully remove slow cooker’s ceramic insert, leaving cover on, and rotate insert 180 degrees. Continue to cook on High heat setting 45 to 60 minutes or until toothpick inserted in center comes out clean. Uncover, and transfer ceramic insert from slow cooker to cooling rack. Let stand 10 minutes.
4Using parchment paper, carefully lift cake out of ceramic insert, and transfer to cooling rack. Remove parchment paper. Cool completely, about 1 hour.