// https://www.reddit.com/r/FastLED/comments/16k4st2/transition_from_one_colour_to_another_using_for/

#include <FastLED.h>

#define NUM_LEDS 24
CRGB leds[NUM_LEDS];

void setup() {
  FastLED.addLeds<WS2812B, PB1, GRB>(leds, NUM_LEDS);
}

void loop() {
  CRGB colA = 0xffd500;
  CRGB colB = 0x00bf60;

  for (int i = 0; i < 256; i++) {
    CRGB col = colA;
    fadeTowardColor(col, colB, i);
    fill_solid(leds, NUM_LEDS, col);
    FastLED.show();
    delay(10);
  }
}

// The code below is from Mark Kriegsman's examples
// https://gist.github.com/kriegsman/d0a5ed3c8f38c64adcb4837dafb6e690

// 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;
}