How do I change the placeholder text color on a form

You will need to add custom CSS to your form in order to set the placeholder text color.  Adding the following CSS will set the placeholder text to be the same color and the input text.  You can then adjust the color property to change the placeholder text color to anything else you choose.

%FORM.HTML_ID% ::-webkit-input-placeholder { /* Chrome/Opera/Safari */
  color: inherit;
}
%FORM.HTML_ID% ::-moz-placeholder { /* Firefox 19+ */
  color: inherit;
}
%FORM.HTML_ID% :-ms-input-placeholder { /* IE 10+ */
  color: inherit;
}
%FORM.HTML_ID% :-moz-placeholder { /* Firefox 18- */
  color: inherit;
}