Vdd = 150 ma, that absolute maximum. I'm assuming you want to connect 5 rgb leds to the port extender. That means each port must get less than:
I_port = 150 mA / 15 = 9.4 mA (this assumes all leds can be on at the same time)
If only one of the 3 leds per package is on at a time then:
I_port = 150 mA / 5 = 30 mA
The leds need about 20 mA each. That means the MCP23017 will work without an additional driver if you limit active leds to 5 per port extender. You also need common anode grb leds. Do not get common cathode leds.
Connect the anode lead of all 5 led packages together. That connection goes to +5V of your external power supply. Each red cathode lead connects to a 150 ohm resistor. Each blue and green cathode lead connects to a 91 ohm resistor. The other end of the resistors goes to a separate expander port. The ground of the external power supply goes to MCP23017 ground. The resistor is sized to provide 20 mA to each led.
Just to be clear, each MCP23017 should not have more than 5 leds on at a time. You don't want to get close to the absolute max rating of 150 mA.
@Gerry48all combined outputs of one MCP23017 cannot exceed 150mA sink current and
one port can sorce or sink a max of 25ma
the MCP23017 has 16 Ports