I sent the two separate sets of HSB values to p5.

int toggleSwitch1_STATE = digitalRead(toggleSwitch1_PIN);
  if(toggleSwitch1_STATE == HIGH) {
    HSB1 = stick.ColorHSV(ringHue, saturation, brightness);
    stick.fill(HSB1, 0, 4);
    stick.fill(0, 4, 4);
    hu1 = map(position, 0, 24, 0, 360);
    sat1 = map(sliderPot1_VALUE, 0, 1023, 0, 100);
    bright1 = map(sliderPot2_VALUE, 0, 1023, 0, 100);
  }
  if (toggleSwitch1_STATE == LOW) {
    HSB2 = stick.ColorHSV(ringHue, saturation, brightness);    
    stick.fill(HSB2, 4, 4);
    stick.fill(0, 0, 4);
    hu2 = map(position, 0, 24, 0, 360);
    sat2 = map(sliderPot1_VALUE, 0, 1023, 0, 100);
    bright2 = map(sliderPot2_VALUE, 0, 1023, 0, 100); 
  }
  stick.show();

	Serial.print(hu1);
  Serial.print(",");
  Serial.print(sat1);
  Serial.print(",");
  Serial.print(bright1);
  Serial.print("; ");
  Serial.print(hu2);
  Serial.print(",");
  Serial.print(sat2);
  Serial.print(",");
  Serial.println(bright2);
  
  delay(500);

function serialEvent() {
  inString = serial.readStringUntil("\\r\\n");
  if (inString != null) {
    console.log(inString);
    let colr = split(trim(inString), ",");
    console.log(colr);
    if (colr.length==6) {
      pos1 = Number(colr[0]);
      sat1 = Number(colr[1]);
      bright1 = Number(colr[2]);
      pos2 = Number(colr[3]);
      sat2 = Number(colr[4]);
      bright2 = Number(colr[5]);
      serial.print('x');
    }
  }
}

I filled two rectangles with the different HSB values.

fill(hu1, sat1, bright1);
rect(50, 150, 100, 100);
fill(hu2, sat2, bright2);
rect(250, 150, 100, 100);

I was able to successfully change one rectangle color without changing the other rectangle color.

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