Contact form 7

::-webkit-input-placeholder { /* WebKit browsers */
    color: #000 !important;
    opacity: 1;
}
:-moz-placeholder { /* Mozilla Firefox 4 to 18 */
    color: #000 !important;
    opacity: 1;
}
::-moz-placeholder { /* Mozilla Firefox 19+ */
    color: #000 !important;
    opacity: 1;
}
:-ms-input-placeholder { /* Internet Explorer 10+ */
    color: #000 !important;
    opacity: 1;
}

Visto en https://codeless.co/contact-form-7-placeholder-text-color/