How to change the color of a Contact Form 7 placeholder text

Add this CSS code to theme options -> styling options -> custom css:

::-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;
}