You have to send it different data for each color.
"TM1803 adopts single wire to communicate and RZ (return to zero code) method to sent signal. On power-on resert status, when chip receive complete 24bits data from DIN, it begin transmitting data to next chip via DO. Before transmission, DO will be keep low-level.OUTR, OUTG, OUTB these 3 PWM will output different duty signal according to different data per 24bits, the cycle of signal is 4ms.If input signal is RESET, the chip will be ready to receive new data after displaying all the received data.The same when receive new 24bit data completely, it will transmit them to next chip via DO."
Read the datasheet - send it 3 bytes, 1 for each color. If you have multiple LEDs/TM1803s, then you will send it multiple groups of 3 bytes.