24bit color

while (( red++ < 256 )) { 
  while (( green++ < 256 )) { 
    while (( blue++ < 256 )) { 
      echo -e -n "\033[48;2;$[ $red-1 ];$[ $green-1 ];$[ $blue-1 ]m " 
    }
    blue=0
  }
  green=0
}
red=0
blue=0
green=0