.rainbowmybb {
  animation: colorRotate 6s linear 0s infinite;
  font-size:14px;
  text-shadow: -1px -1px -1px #ff0000;
  font-weight: bold;
  background: url(https://i.imgur.com/tFGy9bE.gif) 0;
}

@keyframes colorRotate {
  from {
    color: #f5071b;
  }
  10% {
    color: #0099ff;
  }
  50% {
    color: #00ff00;
  }
  75% {
    color: #ab0f1c;
  }
  100% {
    color: #f5071b;
  }
}