Edit in JSFiddle

function mudaCor(novaCor) {
    var x = document.getElementById("demo");
    x.style.color = novaCor;
  }

<h2 id="demo">Este texto pode ser azul ou vermelho!</h2>   
<br>

<a href="#" class="btn" onclick="mudaCor('blue');">Texto azul</a>    
<a href="#" class="btn" onclick="mudaCor('red');">Texto vermelho</a>     

.btn {
  background: #3498db;
  background-image: -webkit-linear-gradient(top, #3498db, #2980b9);
  background-image: -moz-linear-gradient(top, #3498db, #2980b9);
  background-image: -ms-linear-gradient(top, #3498db, #2980b9);
  background-image: -o-linear-gradient(top, #3498db, #2980b9);
  background-image: linear-gradient(to bottom, #3498db, #2980b9);
  -webkit-border-radius: 28;
  -moz-border-radius: 28;
  border-radius: 28px;
  font-family: Arial;
  color: #ffffff;
  font-size: 20px;
  padding: 10px 20px 10px 20px;
  text-decoration: none;
}

.btn:hover {
  background: #3cb0fd;
  background-image: -webkit-linear-gradient(top, #3cb0fd, #3498db);
  background-image: -moz-linear-gradient(top, #3cb0fd, #3498db);
  background-image: -ms-linear-gradient(top, #3cb0fd, #3498db);
  background-image: -o-linear-gradient(top, #3cb0fd, #3498db);
   background-image: linear-gradient(to bottom, #3cb0fd, #3498db);
   text-decoration: none;
 }