Sidebar in Posts Section Only

I’m using the Multi-Color theme and have the sidebars option selected to none. However, I’d like to have a sidebar on just the Posts page (not set as my front/home page). I can get the sidebar to show up on the this Posts page, but not as a true sidebar since it only shows up at the top or bottom of the page. Is there an easy way to have a sidebar displayed for only the Posts page, while still maintaing the full width of the content for the rest of my pages?

I alternately have tried adding a single, right sidebar as part of the theme and deleting the sidebar in the code from all but the Posts page. However, when doing this it shrinks my content width as if the sidebar were still there. Is this the better path to take and then possibly create a new custom page template? I’m guessing it’s the style sheet which is throwing me off.

Any help is greatly appreciated.