Slightly more fancy version with auto-off

nyan.ino

#define AUTO_OFF 1 /* minutes */
unsigned int lastOn = 0;

void setup() {
    Spark.function("shoutRainbows", nyanHandler);
}

void loop() {
    //nothing to see here!
    if (lastOn > 0) {
        float elapsed = ((millis() - lastOn) / 1000.0) / 60.0;
        if (elapsed >= AUTO_OFF) {
            nyanHandler("off");
        }
    }
    
}

int nyanHandler(String cmd) {
    if (cmd == "on") {
        Spark_Signal(true);
        lastOn = millis();
    }
    else if (cmd == "off") {
        Spark_Signal(false);
        lastOn = 0;
    }
}