This rainbow ribbon Jello is one of my grandma's signature desserts - she always makes it for Easter and other spring and summer parties. She layers the colors of the rainbow, but you could certainly adapt it to whatever colors you'd like!
Get 6 small bowls out to start. Mix 1 box of Jello with 1 cup boiling water and ½ cup cold water. Do this with all of the colors at the same time.
Heat the milk in a saucepan over low heat just until it reaches a lukewarm temperature. Meanwhile, in a large bowl, mix together 1 cup boiling water with the envelopes of Knox gelatin. Whisk together until the gelatin is completely dissolved. Add the warm milk, yogurt, sugar and vanilla extract and whisk until thoroughly combined and completely smooth with no lumps.
Pour the first color into a 9x13-inch pan and refrigerate until set (about 45 minutes, but can vary depending on the temperature of your fridge and how full it is). Once it's set, very gently pour 1½ cups of the yogurt mixture on top. Return to the refrigerator for 45 minutes, or until set. Repeat with the colors and yogurt mixture until the last color is used (you will have some yogurt mixture leftover). When finished, refrigerate for at least another hour. Store in refrigerator until ready to serve; refrigerate leftovers.