fail to change placeholder color with Bootstrap 3

Assign the placeholder to a class selector like this:

.form-control::-webkit-input-placeholder { color: white; }  /* WebKit, Blink, Edge */
.form-control:-moz-placeholder { color: white; }  /* Mozilla Firefox 4 to 18 */
.form-control::-moz-placeholder { color: white; }  /* Mozilla Firefox 19+ */
.form-control:-ms-input-placeholder { color: white; }  /* Internet Explorer 10-11 */
.form-control::-ms-input-placeholder { color: white; }  /* Microsoft Edge */

It will work then since a stronger selector was probably overriding your global. I'm on a tablet so i cant inspect and confirm which stronger selector it was :) But it does work I tried it in your fiddle.

This also answers your second question. By assigning it to a class or id and giving an input only that class you can control what inputs to style.

A Possible Gotcha

Recommended Sanity Check - Make sure to add the form-control class to your inputs.

If you have bootstrap css loaded on your page, but your inputs don't have the
class="form-control" then placeholder CSS selector won't apply to them.

Example markup from the docs:

I know this didn't apply to the OP's markup but as I missed this at first and spent a little bit of effort trying to debug it, I'm posting this answer to help others.

There was an issue posted here about this: https://github.com/twbs/bootstrap/issues/14107

The issue was solved by this commit: https://github.com/twbs/bootstrap/commit/bd292ca3b89da982abf34473318c77ace3417fb5

The solution therefore is to override it back to #999 and not white as suggested (and also overriding all bootstraps styles, not just for webkit-styles):

.form-control::-moz-placeholder {
  color: #999;
}
.form-control:-ms-input-placeholder {
  color: #999;
}
.form-control::-webkit-input-placeholder {
  color: #999;
}

I'm using Bootstrap 4 and Dennis Puzak's solution does not work for me.

The next solution works for me

.form-control::placeholder { color: white;} /* Chrome, Firefox, Opera*/
:-ms-input-placeholder.form-control { color: white; }  /* Internet Explorer*/
.form-control::-ms-input-placeholder { color: white; }  /* Microsoft Edge*/