Cách sửa thông thường

.form-control::-webkit-input-placeholder { /* WebKit (Chrome/Opera/Safari), Blink, Edge */
    color: #C7C7CD; /* #909 */
}

.form-control:-moz-placeholder { /* Mozilla Firefox 4 to 18 */
    color: #C7C7CD; /* #909 */
    opacity: 1;
}

.form-control::-moz-placeholder { /* Mozilla Firefox 19+ */
    color: #C7C7CD; /* #909 */
    opacity: 1;
}

.form-control:-ms-input-placeholder { /* Internet Explorer 10-11 */
    color: #C7C7CD; /* #909 */
}

.form-control::-ms-input-placeholder { /* Microsoft Edge */
    color: #C7C7CD; /* #909 */
}

.form-control::placeholder { /* Most modern browsers support this now. */
    color: #C7C7CD; /* #909 */
}

Default color of Placeholder

The colour is #C7C7CD (r: 199 g:199 b: 205) (as what pterry26 said)

and the font-family is HelveticaNeue-Medium and size is 16

Change color of Placeholder

https://stackoverflow.com/questions/2610497/change-an-html5-inputs-placeholder-color-with-css

::-webkit-input-placeholder { /* Chrome/Opera/Safari */
  color: pink;
}
::-moz-placeholder { /* Firefox 19+ */
  color: pink;
}
:-ms-input-placeholder { /* IE 10+ */
  color: pink;
}
:-moz-placeholder { /* Firefox 18- */
  color: pink;
}

Thay đổi màu của placeholder

::placeholder { /* Chrome, Firefox, Opera, Safari 10.1+ */
    color: red;
    opacity: 1; /* Firefox */
}

:-ms-input-placeholder { /* Internet Explorer 10-11 */
    color: red;
}

::-ms-input-placeholder { /* Microsoft Edge */
    color: red;
}