You can adjust message's colors here:
.comments-form-message-error {background: #FFD86F url(images/jc_error.gif) no-repeat 4px 50%}
.comments-form-message-info {background: #DFEEFF url(images/jc_info.gif) no-repeat 4px 50%}
Regarding displaying time... It could be changed only in JS... /components/com_jcomments/js/jcomments-v2.1.js:
this.fadeTimer=setTimeout(function(){JComments.prototype.fade('comments-form-message', 100, 0, 1000);}, 6000);
Change 6000 to greater value...