That’s the worst. With clients, colors and fonts should be off-limits—that’s literally what they’re paying the designer to decide. Clients want control, but they can get this from decisions other than colors and fonts (let them pick target demographic, emotional goals, and establish the business problems the design should solve or speak into).
You can back font and color choices up with market research and objective reasoning to your decisions. But dealing with a client who just won’t listen and demands to pick everything his/herself is a whole other ball game.