Here is some CSS to over ride the styles of the Gravity Forms placeholder colour if it doesn’t change using the Gravity Form Styles plugin…
/* gform placeholders*/
::-webkit-input-placeholder { color:#42210b; }
:-moz-placeholder { opacity: 1; color:#42210b; } /* Firefox 18- */
::-moz-placeholder { opacity: 1; color:#42210b; } /* firefox 19+ */
:-ms-input-placeholder { color:#42210b; } /* ie */
input:-moz-placeholder { color:#42210b; }