Looping Animation of text color change using CSS3

Use keyframes and animation property

p {
  animation: color-change 1s infinite;
}

@keyframes color-change {
  0% { color: red; }
  50% { color: blue; }
  100% { color: red; }
}

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CSS With Prefixes

p {
    -webkit-animation: color-change 1s infinite;
    -moz-animation: color-change 1s infinite;
    -o-animation: color-change 1s infinite;
    -ms-animation: color-change 1s infinite;
    animation: color-change 1s infinite;
}

@-webkit-keyframes color-change {
    0% { color: red; }
    50% { color: blue; }
    100% { color: red; }
}
@-moz-keyframes color-change {
    0% { color: red; }
    50% { color: blue; }
    100% { color: red; }
}
@-ms-keyframes color-change {
    0% { color: red; }
    50% { color: blue; }
    100% { color: red; }
}
@-o-keyframes color-change {
    0% { color: red; }
    50% { color: blue; }
    100% { color: red; }
}
@keyframes color-change {
    0% { color: red; }
    50% { color: blue; }
    100% { color: red; }
}

I have a small css, you can use this

body {
  background-color: #333;
  display: flex;
  justify-content:center;
  align-items: center;
  height: 100vh;
}

.text-rainbow-animation {
  font-family:arial black;
  font-size:70px;
  background-image: 
    linear-gradient(to right, red,orange,yellow,green,blue,indigo,violet, red); 
  -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;  
  animation: rainbow-animation 35s linear infinite;
}

@keyframes rainbow-animation {
    to {
        background-position: 4500vh;
    }
}

<div class="text-rainbow-animation">RAINBOW TEXT</div>