If you have your brand colour codes
If you have hex codes for your brand colours, you can add them straight in to the boxes. If you have RBG or HSL, click the colour box and switch to the one you need from the bottom up/down arrows, then you can enter the details in the boxes.
If you would like to use your brand colours but are not sure of the codes
You can use a colour picker tool to get the colour codes from your website or a document. Just google colour picker and you should find an image colour picker you can use. Once you have the codes you can follow the steps above for 'if you have your brand colours'.
What are Primary and Secondary Colours?
The primary colour will be the colour you would like to be the main colour on your designs. This will usually be the main accent colour of your brand, not your background colour.
The secondary colour will be the second accent colour. in your branding.
Don't worry, you can change these in your settings at any time.