Bootstrap has 3 lines of CSS, within your bootstrap.css generated file that control the placeholder text color:
.form-control::-moz-placeholder {
color: #999999;
opacity: 1;
}
.form-control:-ms-input-placeholder {
color: #999999;
}
.form-control::-webkit-input-placeholder {
color: #999999;
}
Now if you add this to your own CSS file it won't override bootstrap's because it is less specific. So assmuning your form inside a then add that to your CSS:
form .form-control::-moz-placeholder {
color: #fff;
opacity: 1;
}
form .form-control:-ms-input-placeholder {
color: #fff;
}
form .form-control::-webkit-input-placeholder {
color: #fff;
}
Voila that will override bootstrap's CSS.