The Rainbow Riddle (1946) by Margaret Sutton
Judy and Lorraine Lee are getting married in a double wedding outdoor ceremony. Rainbows abound in this story as it rains early, but clears off just in time--with a double rainbow display--for the brides to have the ceremony they dreamed of. But all is not romance and wedding vows. Roberta, a girl whom Judy met in a previous adventure, arrives just before the ceremony bearing a warning that one of the wedding gifts has an explosive in it! And that there are bad men associated with her (Roberta's) aunt who want to get rid of Judy and her soon-to-be husband, Peter, because they're going to interfere with "the rainbow ring."
Judy has no idea what Roberta's talking about and the girl has had a tendency to embroider stories before, so she's reluctant to believe that a gift bearing the address of her good friends Irene and Dale would have an explosive radio in it. However, after the ceremony, Roberta is seen carrying a package into the woods and immediately there is an explosion. It seems she was right after all and fortunately no one was hurt when the package exploded.
But why do these people think Judy and Peter are going to meddle in their business? And what exactly is the business, anyway? Strange incidents follow the couple when they go on their honeymoon. A honeymoon itinerary that has been set by Peter's future boss, Mr. Trent with the FBI. They find themselves stopping at various places with names from the colors of the rainbow. And more explosions happen at various factories. Judy and Peter discover what the rainbow ring is and who is behind it by the time their honeymoon comes to an end.
Another fun entry in the Judy Bolton (now Dobbs) series. Exciting adventures and mystery. Lots of coincidences. And our heroine and her hero have a calm, happy ending--until the next book! ★★★
First line: "Judy! Are you awake?"
Last lines: "Remember what you said about unexpected happiness at the end of the rainbow?" he whispered. "Well, sweetheart, this is it." [You'd think it was a romance novel and not a girl detective mystery series.]