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);
}

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)