$("#topic_content_input[data-placeholder]:not([data-div-placeholder-content]):before").css({"color":"blue"});

<div id="topic_content_input" contenteditable="true" data-placeholder="I want to change my color on click " ></div>

<style>
    #topic_content_input[data-placeholder]:not([data-div-placeholder-content]):before {
        content: attr(data-placeholder);
        float: left;
        margin-left: 2px;
        color: rgba(134,134,134,0.6);
    }
</style>