The placeholder text is set in the HTML, so the only way to override it in CSS is to hide it and use a :before (though maybe the composer input placeholder text should be a setting in the admin?)
So to hide the current text and display your own (in a custom color) you’d need
.wmd-input::-webkit-input-placeholder { color: #fff; }
.wmd-input::-moz-placeholder { color: #fff; }
.wmd-input:-ms-input-placeholder { color: #fff; }
.wmd-input::-webkit-input-placeholder:before { content: 'your content here'; color: salmon; }
.wmd-input::-moz-placeholder:before { content: 'your content here'; color: salmon; }
.wmd-input:-ms-input-placeholder:before { content: 'your content here'; color: salmon; }