The background colors on the "Last Post" column are used to identify the age of the last reply to the topic or subthread. Depending on the theme you're using there can be up to three different colors.

•One for topics updated today
•One for topics updated this week
•One for topics older than a week

In the "default theme" those colors are pink, light yellow and white:



In the "dark theme" those colors are light red, dark blue and black:



In the "dimaging theme" they're light blue, dark blue and black:



If you're not using one of the above themes, you can click here to access the main theme settings page, select the theme you're using and view the settings for topic age.