A function for fading one RGB color toward a target RGB color

fadeTowardColor.ino

#include <FastLED.h>

// fadeTowardColor example code.
//
// Sample code that includes a function for fading one RGB color toward a target RGB color
// Also includes a function for fading a whole array of pixels toward a given color
//
// Both of these functions _modify_ the existing color, in place.
//
// All fades are done in RGB color space.
//
// Mark Kriegsman
// December 2016


#define NUM_LEDS 50
#define LED_PIN 3
#define LED_TYPE WS2811
#define COLOR_ORDER GRB

CRGB leds[NUM_LEDS];

void setup() {
  delay(3000); // sanity delay
  FastLED.addLeds<LED_TYPE,LED_PIN,COLOR_ORDER>(leds,NUM_LEDS);
}




// Helper function that blends one uint8_t toward another by a given amount
void nblendU8TowardU8( uint8_t& cur, const uint8_t target, uint8_t amount)
{
  if( cur == target) return;
  
  if( cur < target ) {
    uint8_t delta = target - cur;
    delta = scale8_video( delta, amount);
    cur += delta;
  } else {
    uint8_t delta = cur - target;
    delta = scale8_video( delta, amount);
    cur -= delta;
  }
}

// Blend one CRGB color toward another CRGB color by a given amount.
// Blending is linear, and done in the RGB color space.
// This function modifies 'cur' in place.
CRGB fadeTowardColor( CRGB& cur, const CRGB& target, uint8_t amount)
{
  nblendU8TowardU8( cur.red,   target.red,   amount);
  nblendU8TowardU8( cur.green, target.green, amount);
  nblendU8TowardU8( cur.blue,  target.blue,  amount);
  return cur;
}

// Fade an entire array of CRGBs toward a given background color by a given amount
// This function modifies the pixel array in place.
void fadeTowardColor( CRGB* L, uint16_t N, const CRGB& bgColor, uint8_t fadeAmount)
{
  for( uint16_t i = 0; i < N; i++) {
    fadeTowardColor( L[i], bgColor, fadeAmount);
  }
}


void loop() 
{
  CRGB bgColor( 0, 15, 2); // pine green ?
  
  // fade all existing pixels toward bgColor by "5" (out of 255)
  fadeTowardColor( leds, NUM_LEDS, bgColor, 5);

  // periodically set random pixel to a random color, to show the fading
  EVERY_N_MILLISECONDS( 300 ) {
    uint16_t pos = random16( NUM_LEDS);
    CRGB color = CHSV( random8(), 255, 255);
    leds[ pos ] = color;
  }

  FastLED.show();
  FastLED.delay(10);
}

i'm unclear on the some of the variables. Where do you state the starting color of the leds? What is the variable for the length of time it takes to transition to the bgColor?
Is it possible to remove the set random pixel component?

In your example sketch, fadeTowardColor.ino, You have two functions with the same name:
CRGB fadeTowardColor() that returns a CRGB data type, and
void fadeTowardColor() in the same sketch.

How does this work without error?

@Steve, because its cplusplus.
The functions have different parameter lists, so they are treated as different functions
CRGB fadeTowardColor( CRGB& cur, const CRGB& target, uint8_t amount)
void fadeTowardColor( CRGB* L, uint16_t N, const CRGB& bgColor, uint8_t fadeAmount)