An electronic device such as a wristwatch may be provided with conductive structures. The conductive structures may include a sensor electrode for an electrocardiogram (ECG) sensor. A coating may be disposed on the sensor electrode to reflect particular wavelengths of visible light so that the sensor electrode exhibits a desired color. The coating may include adhesion and transition layers on the sensor electrode, an opaque coloring layer on the adhesion and transition layers, and a thin-film interference filter on the opaque coloring layer. The thin-film interference filter may have an uppermost diamond-like carbon (DLC) layer. The DLC layer may contribute to the color response of the coating while concurrently minimizing noise in ECG waveforms gathered by the ECG sensor using the sensor electrode.