Colors help you highlight what's important in your data. Excel Online recently updated its color palette to be consistent with your other applications.
Now as you move between versions of Excel or work with other Office Online applications, the choices you see in your color palette will be the same. This will provide a better collaboration experience with other users and provide a consistent experience between devices.
If you like this change and want more, we encourage you to vote for advanced features such as "More Colors..." and "Themes" in the Excel Online User Voice Forum.