void changeColor(char control[16]){
  char _red[3], _green[3], _blue[3], _brightness[3], _white[3];
  int red, green, blue, brightness, white, offset;
  offset=1;
  for(int i=0;i<3;i++){
    _red[i]=control[i+offset];
  }
  offset=4;
  for(int i=0;i<sizeof(green);i++){
    _green[i]=control[i+offset];
  }
 // strncpy(_green,control+offset,3); // same result
 offset=7;
 for(int i=0;i<sizeof(_blue);i++){
   _blue[i]=control[i+offset];
 }
 offset=10;
 for(int i=0;i<sizeof(_white);i++){
   _white[i]=control[i+offset];
 }
 offset=13;
 for(int i=0;i<sizeof(_brightness);i++){
   _brightness[i]=control[i+offset];
 }
 red=atoi(_red);
 green=atoi(_green);
//sscanf(_green, "%d", &green); //same result
blue=atoi(_blue);
white=atoi(_white);
brightness=atoi(_brightness);
Serial.print(" control ");
Serial.println(control);
Serial.print(" _red ");
Serial.println(_red);
Serial.print(" _green ");
Serial.println(_green);
Serial.print(" _blue ");
Serial.println(_blue);
Serial.print(" _white ");
Serial.println(_white);
Serial.print(" _brightness ");
Serial.println(_brightness);
Serial.print(" red ");
Serial.println(red);
Serial.print(" green ");
Serial.println(green);
Serial.print(" blue ");
Serial.println(blue);
Serial.print(" white ");
Serial.println(white);
Serial.print(" brightness ");
Serial.println(brightness);
//... do something with ints
}

 char _control[16]={'6','2','5','5','0','0','0','0','0','0','0','0','0','0','5','0'};
 //6255000000000050

 void setup() {
   Serial.begin(9600);
   // ... do something
   while (!Serial) {
     ; // wait for serial port to connect. Needed for native USB
   }
  changeColor(_control);
}

   control 6255000000000050
   _red 255ÿ
   _green 000255ÿ
   _blue 000000255ÿ
   _white 000050000000255ÿ
   _brightness 050000000255ÿ
   red 255
   green 255
   blue 255
   white 29951
   brightness 29951

I also tried with strncpy, strcpy, strncat, and by using the pointer in the for loops

for(int i=0;i<sizeof(_green);i++){
  _green[i]=control+i*sizeof(char)+offset*sizeof(char);
} // also without the sizeof()

  ...
  char _red[4], _green[4], _blue[4], _brightness[4], _white[4];
  int red, green, blue, brightness, white, offset;
  _red[4]='\0';
  _green[4]='\0';
  _blue[4]='\0';
  _white[4]='\0';
  _brightness[4]='\0';
  offset=1;
  for(int i=0;i<3;i++){
    _red[i]=control[i+ offset];
  }
  // same as above

control 6255000000000050
_red 255
_green 100255
_blue 000100255
_white 0000050
_brightness 050
red 255
green 100
blue 0
white 50
brightness 50

void changeColor(char control[16])
{
  char _red[3], _green[3], _blue[3], _brightness[3], _white[3];
  int red, green, blue, brightness, white, offset;
  
  offset = 1;
  for (int i = 0; i < 3; i++) {
    _red[i] = control[i + offset];
  }
  offset = 4;
  for (int i = 0; i < sizeof(green); i++) {
    _green[i] = control[i + offset];
  }
  // strncpy(_green,control+offset,3); // same result
  offset = 7;
  for (int i = 0; i < sizeof(_blue); i++) {
    _blue[i] = control[i + offset];
  }
  offset = 10;
  for (int i = 0; i < sizeof(_white); i++) {
    _white[i] = control[i + offset];
  }
  offset = 13;
  for (int i = 0; i < sizeof(_brightness); i++) {
    _brightness[i] = control[i + offset];
  }
  
  red = atoi(_red);
  green = atoi(_green);
  //sscanf(_green, "%d", &green); //same result
  blue = atoi(_blue);
  white = atoi(_white);
  brightness = atoi(_brightness);
  
  Serial.print(" control ");
  Serial.println(control);
  Serial.print(" _red ");
  Serial.println(_red);
  Serial.print(" _green ");
  Serial.println(_green);
  Serial.print(" _blue ");
  Serial.println(_blue);
  Serial.print(" _white ");
  Serial.println(_white);
  Serial.print(" _brightness ");
  Serial.println(_brightness);
  Serial.print(" red ");
  Serial.println(red);
  Serial.print(" green ");
  Serial.println(green);
  Serial.print(" blue ");
  Serial.println(blue);
  Serial.print(" white ");
  Serial.println(white);
  Serial.print(" brightness ");
  Serial.println(brightness);
  //... do something with ints
}

 char _control[16]={'6','2','5','5','0','0','0','0','0','0','0','0','0','0','5','0'};
 //6255000000000050

 void setup() {
   Serial.begin(9600);
   // ... do something
   while (!Serial) {
     ; // wait for serial port to connect. Needed for native USB
   }
  changeColor(_control);
}

   control 6255000000000050
   _red 255ÿ
   _green 000255ÿ
   _blue 000000255ÿ
   _white 000050000000255ÿ
   _brightness 050000000255ÿ
   red 255
   green 255
   blue 255
   white 29951
   brightness 29951

I also tried with strncpy, strcpy, strncat, and by using the pointer in the for loops

for(int i=0;i<sizeof(_green);i++){
  _green[i]=control+i*sizeof(char)+offset*sizeof(char);
} // also without the sizeof()

  ...
  char _red[4], _green[4], _blue[4], _brightness[4], _white[4];
  int red, green, blue, brightness, white, offset;
  _red[4]='\0';
  _green[4]='\0';
  _blue[4]='\0';
  _white[4]='\0';
  _brightness[4]='\0';
  offset=1;
  for(int i=0;i<3;i++){
    _red[i]=control[i+ offset];
  }
  // same as above

control 6255000000000050
_red 255
_green 100255
_blue 000100255
_white 0000050
_brightness 050
red 255
green 100
blue 0
white 50
brightness 50

EDIT:

Added NULL char

  ...
  char _red[4], _green[4], _blue[4], _brightness[4], _white[4];
  int red, green, blue, brightness, white, offset;
  _red[4]='\0';
  _green[4]='\0';
  _blue[4]='\0';
  _white[4]='\0';
  _brightness[4]='\0';
  offset=1;
  for(int i=0;i<3;i++){
    _red[i]=control[i+ offset];
  }
  // same as above

Output:

control 6255000000000050
_red 255
_green 100255
_blue 000100255
_white 0000050
_brightness 050
red 255
green 100
blue 0
white 50
brightness 50