Hello Antonio,
Try these codes and see if it works for you. You need to change the color accordingly.
body div#fd.es .mentions textarea::-webkit-input-placeholder {
/* Setting placeholder text color in webkit browser */
color: red;
}
body div#fd.es .mentions textarea:-moz-placeholder {
/* Setting placeholder text color in FF 4-18 */
color: red;
}
body div#fd.es .mentions textarea::-moz-placeholder {
/* Setting placeholder text color in FF 19+ */
color: red;
}
body div#fd.es .mentions textarea:-ms-input-placeholder {
/* Setting placeholder text color in IE 10+ */
color: red;
}
body div#fd.es .mentions textarea {
/* Setting textarea text color during user writing their story. OPTIONAL */
color: red;
}