Un sencillo mixin SCSS para darle color al texto del placeholder, distinto del que lleve el campo de formulario:

@mixin input-placeholder($color) {
  &.placeholder {color: $color;} /* if not supported, Javascript should add class */
  &:-moz-placeholder {color: $color;}  /* Mozilla Firefox 4 to 18 */
  &::-moz-placeholder {color: $color;}  /* Mozilla Firefox 19+ */
  &::-webkit-input-placeholder {color: $color;} /* WebKit browsers */
	&:-ms-input-placeholder {color:$color;} /* Internet Explorer 10+ */

  /* browsers with full support for placeholder styling */
  &::input-placeholder {color: $color;}
  &:input-placeholder {color: $color;}
  &::placeholder {color: $color;}
  &:placeholder {color: $color;}
}