how to change color of orange line at bottom of posts

for those general formatting problems, try working with a browser inspection tool such as Firefox’ add-on Firebug or similar.

this should help you to identify where in the stylesheets the border and color gets defined;

in your case this would be in style.css of your theme, this style:

body.single article[id*=post-],
			body.page article[id*=post-] {
				border-bottom: 2px solid #f60;
				margin-bottom: 30px;
			}