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.