test.py

import serial
import time

DEVICE = '/dev/ttyUSB0'
SPEED = '115200'
PWM_MAX = 20

def get_data(color, value):
    assert color in 'RGB'
    assert 0 <= value <= PWM_MAX
    cbit = 'RGB'.index(color)
    data = cbit << 6 | value
    return chr(data)

def write_color(ser, color, value):
    data = get_data(color, value)
    ser.write(data)

def main():
    ser = serial.Serial(DEVICE, SPEED)
    ser.setDTR()
    ser.flushInput()
    ser.flushOutput()

    write_color(ser, 'R', 0x0f)
    time.sleep(0.5)
    write_color(ser, 'B', 0x0f)
    time.sleep(0.5)
    write_color(ser, 'R', 0x00)
    time.sleep(0.5)
    write_color(ser, 'B', 0x00)

if __name__ == '__main__':
    main()