HTML:

<input type="text" placeholder="sample text"/>

CSS:

input::-webkit-input-placeholder {
    color: #999;
}
input:focus::-webkit-input-placeholder {
    color: red;
}
/* Firefox < 19 */
input:-moz-placeholder {
    color: #999;
}
input:focus:-moz-placeholder {
    color: red;
}
/* Firefox > 19 
 
*/
input::-moz-placeholder {
    color: #999;
}
input:focus::-moz-placeholder {
    color: red;
}

/* Internet Explorer 10 */
input:-ms-input-placeholder {
    color: #999;
}
input:focus:-ms-input-placeholder {
    color: red;
}

input獲得焦點時改變placeholder文本的樣式