One of the major changes in the new material settings is the way colors are added to materials. Formerly we would treat every color as a separate material, now it has become a property of that material, which reflects the practice of selling different colors of the same filament for the same price. 

Colors are now added in the "color" section of the material, just like layer thickness or filling density. To add a color you can either select one from the list available: 

As you can see the selected color appears below the selection field.

Or you can simply start writing the name of your own color, let's say terracotta:
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Once you've finished writing you click "Create terracotta"
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and the new color is added. You can click on the white box next to the color name to select the color using a manual color pallet or by using the color code of your choice.  

This is what it will look like on the widget: 

If you have a lot of color options the colors can also be displayed in a drop-down menu.