Hello, You can add following in additional CSS in customizer:
.ur-frontend-form form .ur-form-row .ur-form-grid input[type="text"]::-webkit-input-placeholder {
color: #000; }
.ur-frontend-form form .ur-form-row .ur-form-grid input[type="text"]::-moz-placeholder {
/* Firefox 19+ */
color: #000; }
.ur-frontend-form form .ur-form-row .ur-form-grid input[type="text"]:-ms-input-placeholder {
/* IE 10+ */
color: #000;}
.ur-frontend-form form .ur-form-row .ur-form-grid input[type="text"]:-moz-placeholder{
/* Firefox 18- */
color: #000; }
That is for text input type, similarly you can change for other types.