FastLED - How to Control specific LEDs
The colors of the LEDs in your sketch are set in this loop:

for(int i = 0; i &lt; NUM_LEDS; i++ )
{
    leds[i].setRGB(0,255,250);  // Set Color HERE!!!
    leds[i].fadeLightBy(brightness);
}


You loop over all LEDs and set them to a specific color and brightness. The changes will be send to the LEDs, when you call FastLED.show() afterwards. There is nobody, that prevents you from setting a different color for each LED or to only chnage the value for some of the LEDs.

For example, to light the first 5 LEDs in red, the next 10 in green and leaving the rest as they were before, you can use this code (Just exchange it with your for loop):

for(int i=0;i&lt;5;i++){
    leds[i].setRGB(255,0,0);
    leds[i].fadeLightBy(brightness);
}
for(int i=5;i&lt;15;i++){
    leds[i].setRGB(0,255,0);
    leds[i].fadeLightBy(brightness);
}
FastLED.show();


The principle is, that you have an array with color values. Each value consists of 3 smaller values for red, green and blue (RGB)(The color results from the distribution between red, green and blue; the brightness results from the overall level of red, green and blue). And each value of the array is bound to an LED (First value to first LED, second value to second LED,...). You can change the color values in the array as you like. And when you want the LEDs to show the current values in the array, you send them out by using FastLED.show(). This means you have to call this function everytime, that you want LEDs to change. You don't have to change every value in the array for this to work.

If you would like to build an asynchronous change of colors, you can ditch the delay(80) and use non-blocking code like in the BlinkWithoutDelay example of the Arduino IDE. There you use the millis() function as a clock (it returns the milliseconds since the Arduinos startup). You take a timestamp at the beginning and compare the difference between it and the current time:

unsigned long timestamp=0;
unsigned long interval=80; //interval of 80 milliseconds

void loop(){
    unsigned long current_time=millis();
    if(current_time - timestamp &gt; interval){
        // do something here, like setting LED colors
        timestamp = current_time;
        // don't forget FastLED.show(), to output the colors
    }
}


You can use multiple of these timestamps with if-statements, one for each segment you want to change in a different frequency.