.mydiv {
  width:100%;
  height:100%;
  color:black;
  font-weight:bold;
  animation: myanimation 10s infinite;
}

@keyframes myanimation {
  0% {background-color: red;}
  25%{background-color:yellow;}
  50%{background-color:green;}
  75%{background-color:brown;}
  100% {background-color: red;}
}