Hi TheoL,
I have changed the value from 0 to 255 as shown below and now the LED strip turn off when I click the off button in domoticz.
Now the dimming part has yet to be solved :)

Was
if (message.type == V_LIGHT) {
if (message.getInt() == 0) {
digitalWrite(RED_PIN, 255);
digitalWrite(GREEN_PIN, 255);
digitalWrite(BLUE_PIN, 255);```