How can I delete the think-cell custom colors?

think-cell adds the most recently used custom colors to the color control for quick access (see 3.8.1 Color and fill). The colors to show in the color control are saved in two places:

• In the current presentation, so that other people editing the same presentation have the custom colors available.
• On your computer, so that you have the custom colors available in other (e.g., new) presentations.

There is currently no official means to clear the custom colors list. While there is no way at all to remove custom colors from a presentation file, the part of the list that is stored on your computer (the part below the divider line) can in fact be cleared. However, be aware that you will lose some other settings, too, including your custom precisions list and possibly your license key (which you can re-enter on the next start of PowerPoint or Excel).