How do I stop my robot after it drives forward over a black band?

%%sim_magic_preloaded --background coloured_bands -R

Program to count the bands aloud

Start the robot moving

tank_drive.on(SpeedPercent(15), SpeedPercent(15))

Initial count value

count = 0

Initial sensor reading

previous_value = colorLeft.reflected_light_intensity_pc

Create a loop

while True:

# Check current sensor reading
current_value = colorLeft.reflected_light_intensity_pc

# Test when the robot has entered a band
if previous_value==100 and current_value < 100:
    # When on a new band:
    # - increase the count
    count = count + 1
    # - display the count in the output window
    # - say the count aloud

# Update previous sensor reading
previous_value = current_value