Recent colors are handled inconsistently by Microsoft Office programs. Word and Outlook only retain recently used colors only as long as the program is running, and those colors are visible in every document that's open. By contrast, PowerPoint and Excel both include them in the document. As soon as you open a different file, the previous colors disappear from the color picker. Return to the first document and there they are again.
If you're creating files for clients, you may generate quite a few colors in the design process. Your work will look a little more professional if you purge the Recent Colors from the PowerPoint or Excel file before sending it on.
If you're new to XML hacking, please read my intro: XML Hacking: An Introduction. If you're working on a Mac, you should also read XML Hacking - Editing in macOS.
The row of Recent Colors that PowerPoint and Excel include in the file is a distraction for your client.
Recent Colors Removal Steps
Under Windows, begin by unzipping the file. On a Mac, open it in BBEdit or other advanced text editor.
If this is a PowerPoint deck, look in the ppt folder for the presProps.xml file. The recent colors begin on the third line of a prettified (human-readable) file. Simply delete the entire clrMru section:
To remove the Recent Colors from an Excel workbook, open the xl folder, then edit styles.xml. Look for the colors section, then delete the entire mruColors part:
After editing, rezip the files if you're using Windows or save and close in your macOS text editor. Test that the file opens as expected and no longer has Recent Colors in the color picker, that send it off to the client.
A more presentable dialog for your client.
Don't Reuse Recent Colors
Occasionally an artist will try to include special colors in the Recent Colors section, to give the client some additional color choices. This is a bad idea because the Recent Color section is dynamic. When any new color is created, it's added to the left end of Recent Colors. If the row is full, the oldest color gets pushed off the right end. A better solution is to create Custom Colors. Here's my how-to on the subject. Custom Colors don't move or change and can be named, which is a little extra help for your client.