What happens if Bob has a Blue Filter, and looks at a: 1. A Blue Case 2. a Red Hat

1. Since a Blue Filter allows the Blue Wavelength, and the Case is Blue, This means the Case will still be Blue because its still allowing the Reflected Blue to hit the Eyes 2. For the Red Hat, the Reflected Red Wavelength would be Blocked [Absorbed] by the Filter, making it look Black [As no Light can Penetrate the Filter]