This option allows you to set the color of the embed. The color is visible on the left side of the embed. When you click the option, you will get a select menu with different options like:
This option allows you to set the color of the embed to a custom color. When you click the option, you will get a form where you need to fill in the HEX code of the color.You can find the HEX code of a color on this site.
Set Color Custom Hex Code Form
Once you submit the information, the embed is updated.