The Colors tab is located under Design in the Design Panel. There you can use and edit existing color palettes.
Using a Color Palette
The Palettes area contains pre-made palettes for the background, text and other elements of your Goodsie store. To activate a palette, simply click on its name.
Editing a Custom Palette
To edit an existing palette, click on Custom, then click on the color that you would like to change within the Backgrounds, Page or Product areas. A color selection tool will appear. Select the color you’d like to use or enter a custom hexadecimal and click Apply, then Save. You can also change the opacity (/transparency) of background elements here, using the opacity slider under the color selector.