A portable physical activity monitor device includes a power source for providing power to the physical activity monitor device, at least one processor and at least one memory including a computer program code. The at least one memory and the computer program code are configured, with the at least one processor, to cause the physical activity monitor device to perform operations including acquiring an indication of a physical activity metric of a person carrying the physical activity monitor device. The physical activity monitor device includes at least one electrochromic part including bi-stable electrochromic material, and the physical activity monitor device is caused to control an optical characteristic.
