On the Designer, there are a few different places where you will be able to select colours or gradients. One of these places is on the Fill Tool...

Colour Swatches make it easy to save a specific colour so that it can be applied to multiple elements easily, or even just to build a consistent colour palette for your Cake Design.

To save a colour to the swatches, simply select a colour from the colour picker, and then click on the + symbol at the end of the swatches. This will add the current colour to the swatches. 

To remove a colour, simply right click on it. 

The same steps will apply for saving gradients through the linear gradient mode and the radial gradient mode. 

Note: Swatches are saved as part of a Design. At this time, there is no way to use a collection of swatches from one Design, on another/new Design.