You can change an HTML5 input's placeholder color with CSS. If by chance, your CSS conflict, this code note working , you can use (!important) like below.
::-webkit-input-placeholder { /* WebKit, Blink, Edge */
color:#909 !important;
}
:-moz-placeholder { /* Mozilla Firefox 4 to 18 */
color:#909 !important;
opacity:1 !important;
}
::-moz-placeholder { /* Mozilla Firefox 19+ */
color:#909 !important;
opacity:1 !important;
}
:-ms-input-placeholder { /* Internet Explorer 10-11 */
color:#909 !important;
}
::-ms-input-placeholder { /* Microsoft Edge */
color:#909 !important;
}
<input placeholder="Stack Snippets are awesome!">
Hope this will help.