When using the HTML editor you have the option to use your own code to add a button, upload a button image and then link it, or you can use the 'Simple Button' option in the toolbar to create a button.
Once clicked you will see options for Text of the button, the URL when clicked, the Font Size, the Border Radius, the Background color options as well as a Preview.
If you know you your color code you can enter it in the field or you can use the grid to select a color as shown below and you will see a menu of additional colors to choose from.
Once done select 'OK' and this will insert your button to the HTML Editor.
If you would like to edit the button just double click it to get back into the properties.
That was easy, wasn't it? Let us know if you have any questions!