Hej Hans,

tack så mycket! I need coffee to get me through this quest, which on paper sounds so very simple and straightforward. You could upload the code above, which works well), to see what actually happens (a 10 minute version of what later will take 3 hours) and of course maybe change the 10 minute value to only 2 minutes to not fall asleep : )

What the above does is lighting an entire 144 LED strip with colours fetched from a self-made palette (later, there will be three palettes - sunrise, daytime, sunset) or paletteknife palette (it is quite simple to create one's own palettes rather than using the built-in "disco palettes").

So, In parallel, while the above goes on ever so slowly, I cannot figure out how in addition, with a different timing (faster, but not very fast), random LEDs can be randomly dimmed up/down (between 128 and 255 brightness) to achieve the aforementioned "shimmering" effect.

Meanwhile, I think the code principle below (I used a very fast "shimmering" effect just to have something a bit similar to look at) with two independent calls to EVERY_N_MILLISECONDS could be the right framework to have two effects going on independently from each other, but affecting the same LED array.

What you suggest makes sense (trying nscale8) and I try it out today, if I find the time; I realised that fadeToBlackBy is not right for what I'm after, because that turns off LEDs fully.

Thanks a lot for your input!

#include <FastLED.h>
const byte pinData = 3;
const byte pinClock = 4;
const byte ledCount = 144;
byte maxBrightness = 255;
struct CRGB leds[ledCount];
void setup() {
  LEDS.addLeds<APA102, 3, 4, BGR>(leds, ledCount);
  LEDS.setBrightness(maxBrightness);
  LEDS.setCorrection(Candle); // Candle too warm, Tungsten40W too cold - in-between correction methods?
}
void loop() {
  int ledPosition = random8(ledCount);
  EVERY_N_MILLISECONDS(100) {
    leds[ledPosition] = CHSV(0, 0, random(255));
  }
  EVERY_N_MILLISECONDS(2000) {
    fadeToBlackBy( leds, ledCount, random(128)); // 1 = slow, 255 = fast
  }
  FastLED.show();
  FastLED.delay(100);
  
}

I see I'm not alone when it comes to the need for coffee 

I'd love to give it a try, but I recently moved and all my Arduino gear is still boxed up haha. So testing is a challenge here.
I yet have to play with "EVERY_N_MILLISECONDS", but found this example quite useful, so I think your code is at least going in the right direction.

So in the end all LEDs should have full color by the time you're done with a palette, right?

Thanks, maybe some of the many other FastLED user-coders here come up with something...

The entire strip is fully lit via the palette at all times, as shown in the first code example that works. The shimmer effect should then simply modulate that illumination by way of random brightness changes of random LEDs concurrently. It's really that simple (on paper).

But the FastLED documentation is not very thorough for each basic function and most examples I found online are nearly the same, packed full of bouncy disco effects where one user just seemingly copied from another or modifies things to their liking. But there is no from-the-ground-up explanation online that goes from the basics upwards - addressing LEDs, addressing LEDs randomly, addressing LEDs from external inputs... if you know what I'm trying to say. What I mean is that most examples are extremely convoluted so it is hard to dissect them for essential information, for the "FastLED building blocks", so to speak.

The EVERY_N_MILLISECONDS example is the only one one can find with Google or Bing and there are only around 600 search results, nearly all with the same code, taken from an original demo.

I'm not sure how many coders will be looking at this forum (I hope one there will be more).

What I'd do is start with a simple version of the project, say 4 LEDs and one palette of colors.

On paper it is simple indeed - let's start with a palette we want to dim;

1) Apply Full Palette
2) In a loop, repeat the following steps for a while
3) Pick one or more random LEDs and reduce their brightness 
4) Make changes visible

Step 3 can be done like this; 

i = random(ledCount);
leds = leds.nscale8_video( random(192) );

void loop() {
  int ledPosition;
  
  // Step 1: Apply full palette
  // I think your skyEffect() does this?
  skyEffect();

  // Step 2 the rinse and repeat loop
  for(Counter=0;Counter<255;Counter++) { 
    ledPosition = random8(ledCount);   // Step 3A: random dimming LEDs
    leds[ledPosition] = leds[ledPosition].nscale8_video( random(50) );
    FastLED.show();                    // Step 4B : make the changes visible
    delay(100);                        // just taking a short time interval for testing
 }
}

void loop() {
  int ledPosition;
  
  // Step 1: Apply full palette
  // I think your skyEffect() does this?
  skyEffect();
  // Step 2 the rinse and repeat loop
  do { 
    ledPosition = random8(ledCount);   // Step 3A: random dimming LEDs
    leds[ledPosition] = leds[ledPosition].nscale8_video( random(50) );
    FastLED.show();                    // Step 4B : make the changes visible
    delay(100);                        // just taking a short time interval for testing
  } while !AreAllLEDsDimmed;
}

bool AreAllLEDsDimmed {
  bool AreWeDone = true;
  for(int i; i<ledCount; i++) {
    AreWeDone = AreWeDone && ( max(leds.red, max(leds.green, leds.blue) ) < 50 );
  }
  return AreWeDone;
}

void loop() {
  int ledPosition;
  
  // Step 1: Apply full palette
  // I think your skyEffect() does this?
  skyEffect();
  // Step 2 the rinse and repeat loop
  do { 
    ledPosition = random8(ledCount);   // Step 3A: random dimming LEDs
    
    while(IsThisLEDDimmedEnough(ledPosition) {
      ledPosition = random8(ledCount);
    }
    
    leds[ledPosition] = leds[ledPosition].nscale8_video( random(50) );
    FastLED.show();                    // Step 4B : make the changes visible
    delay(100);                        // just taking a short time interval for testing
  } while !AreAllLEDsDimmed;
}
bool AreAllLEDsDimmed {
  bool AreWeDone = true;
  for(int i; i<ledCount; i++) {
    AreWeDone = AreWeDone && IsThisLEDDimmedEnough(i);
  }
  return AreWeDone;
}

bool IsThisLEDDimmedEnough(int i) {
  return ( max(leds.red, max(leds.green, leds.blue) ) < 50 );
}

ledPosition = random8(ledCount);   // Step 3A: random dimming LEDs
while(IsThisLEDDimmedEnough(ledPosition) {
  ledPosition = random8(ledCount); // pick another LED
}

randomSeed(analogRead(0));