Hello,

Please add this code to the Theme Settings > Custom CSS > Global:

body .wd-search-full-screen .searchform input[type="text"]::placeholder { /* Chrome, Firefox, Opera, Safari 10.1+ */
  color: red;
  opacity: 1; /* Firefox */
}

body .wd-search-full-screen .searchform input[type="text"]:-ms-input-placeholder { /* Internet Explorer 10-11 */
  color: red;
}

body .wd-search-full-screen .searchform input[type="text"]::-ms-input-placeholder { /* Microsoft Edge */
  color: red;
}

Best Regards