A mobile computing device may include a biometric sensor in proximity with a color-controlled layer. Where the color of the color-controlled layer may be changed, a processor of the mobile computing device may control the color of the color-controlled layer responsive to sensing various conditions. For example, the color of the color-controlled layer may be controlled, by default, to match the housing of the mobile computing device. Responsive to sensing an approaching user finger, the color-controlled layer may be controlled to change color. The color-controlled layer may be controlled to change color again upon contact of the finger, upon removal of the finger and upon determining authentication success. The color-controlled layer may be structured as a mix of heating elements and thermochromic polymer elements.