Advantages of Multi-site

Hello
My client has a site where they have 3 categories of content. They want me to create a custom color for each category page. I am not sure if I should create a separate site for each category, rather than keeping each category on the same site.

If I customize the color of each category page, is this easy to do using the WPMU themes available?

Would it be a better idea to create a multi-site?

Dustin