idea for a another overloading function for the Adafruit

Adafruit_NeoPixel::Color(String).ino

#include <Adafruit_NeoPixel.h>
#include <string.h>

class NeoPixel : public Adafruit_NeoPixel
{
  public:
  NeoPixel(uint16_t pixels, uint8_t pin, uint8_t type):Adafruit_NeoPixel(pixels, pin, type){};
    
  uint32_t Color( String col_str ){
    // int to hex - 16 chars + terminator
    char itoh_lower[18] = {"0123456789abcdef"};
    char itoh_upper[18] = {"0123456789ABCDEF"};
    int htoi_number = 0;    // return value
    for(int index = 0; index < col_str.length(); index ++){         // check each hex char in string
      for(int val = 0; val < 16; val ++){                           // against each available hex character
        if (  col_str[col_str.length() -index -1] == itoh_lower[val] ||
              col_str[col_str.length() -index -1] == itoh_upper[val] ){
          if(val > 0){
            htoi_number += (uint32_t)(val * (16 << index));
          }
        }
      }
    }
    return (uint32_t)(htoi_number);
  }
};

SerialToRGBW.ino

#include <Adafruit_NeoPixel.h>
#include <string.h>

class NeoPixel : public Adafruit_NeoPixel
{
  private:
  uint32_t power(uint8_t p_base, uint8_t p_exp){
    uint32_t pow_ret = 1;
    for(int p_ctr = 0; p_ctr < p_exp; p_ctr ++){
      pow_ret *= p_base;
    }
    return pow_ret;
  }
  
  public:
  NeoPixel(uint16_t pixels, uint8_t pin, uint8_t type):Adafruit_NeoPixel(pixels, pin, type){};
    
  // Convert RGB / RGBW string into packed 32-bit WRGB color.
  // string format: "RRGGBB" or "RRGGBBWW (no 0x)
  uint32_t Color( String htoi_str ){
    char itoh_lower[18] = {"0123456789abcdef"};       // 16 chars + null
    char itoh_upper[18] = {"0123456789ABCDEF"};       // 16 chars + null
    uint32_t htoi_number = 0;
    for(uint8_t index = 0; index < htoi_str.length(); index ++){        // each hex number in String
      for(uint8_t val = 0; val < 16; val ++){                           // each possible value
        if (  htoi_str[htoi_str.length() -index -1] == itoh_lower[val] ||
              htoi_str[htoi_str.length() -index -1] == itoh_upper[val] ){
          if(val > 0){
            // htoi_number += val * power(16, index);
            htoi_number += (uint32_t)( val * (power(16.0, index) + 0.01) );
          }
        }
      }
    }
    return (uint32_t)(htoi_number);
  }
};

#define PIN 6
#define NUM_LEDS 16

NeoPixel strip(NUM_LEDS, PIN, NEO_GRB + NEO_KHZ800);

void setup() {
  // put your setup code here, to run once:
  strip.begin();
  strip.show();
  Serial.begin(9600);
  Serial.println("HEX to RGBW");
  Serial.println("type in a HEX color string (RRGGBB or WWRRGBB - no '0x')");
}

void loop() {
  // put your main code here, to run repeatedly:
  String str;
  if(Serial.available() > 0){
    str = Serial.readString();
    if(str.length() == 6){      // == 6 if RGB / == 8 if RGBW
      uint32_t couleur = strip.Color(str);
      for(int i=0; i<NUM_LEDS; i++){
        strip.setPixelColor(i, couleur);
      }
      strip.show();
      Serial.println(str);
      Serial.print("W\t"); Serial.println( ((couleur >> 24) & 0xff));
      Serial.print("R\t"); Serial.println( ((couleur >> 16) & 0xff));
      Serial.print("G\t"); Serial.println( ((couleur >> 8 ) & 0xff));
      Serial.print("B\t"); Serial.println( ((couleur       ) &0xff));
      Serial.println("----------");
    }
  }
}

Serial To RGBW

This sketch converts a String received over Serial to a 32 bit color value to use with the Adafruit NeoPixel library.

colsole output:

HEX to RGBW
type in a HEX color string (RRGGBB or WWRRGBB - no '0x')
ff2005
W	0
R	255
G	32
B	5
----------
000008
W	0
R	0
G	0
B	8
----------