.marquee {
  display: flex;
  align-items: center;
  overflow: hidden;
}

.letter {
  font-size: 14px;
  animation: rainbow 2s linear infinite;
  margin-right: 5px;
}

@keyframes rainbow {
  from {
    color: red;
  }
  
  25% {
    color: orange;
  }
  
  50% {
    color: yellow;
  }
  
  75% {
    color: green;
  }
  
  to {
    color: blue;
  }
}