Themes don’t come with content. Looking at a theme online then seeing it installed can be a bit of a surprise. The theme you download doesn’t have content. So to get a better idea of how your site will look with a specific theme, create a Page, a Post and activate some sidebar widgets. This will provide some content for the theme to display and give you a more accurate representation of the theme.
If you’re new to WordPress, look at several previews of free themes to get an idea what elements of your site will change with a theme. This process will help you become more familiar with what you like and what you don’t like.
The color of a theme can be change with a simple bit of code so don’t focus on that. Focus on the overall layout – does it do what you need.
Once the theme is installed, bring up the various page templates: archives, multi-post pages, single-post pages too see how the layout changes.
If you are just exploring WordPress to see if starting and maintaining a blog is for you, start with a simple free theme.
Focus on writing posts instead of getting bogged down with theme choices.
And remember, if after installing and working with it, you don’t like your existing theme, with three button clicks and you can choose again.