If you are using the QTR Reflectance Sensor library, setting the
white_line argument of the
readLine() function to 1 allows you to detect a white line on a dark background. For more information on how that function works, see its function definition in QTRSensors.cpp in the library.
For the complete documentation of the A-Star 32U4 Arduino Library, including its main and component classes and functions, follow the link under the "Documentation" section on its GitHub page.