In the left panel of the Imagink application, you can select any colour to customize your current brush colour.
To change a preset colour, double click on a colour slot and select a new colour with the help of the colour selector which will open.
To use the colour picker, double click on a preset colour slot and select the button "Pick Screen Color". Move your mouse over the drawing (or the photo you have imported) and click once when having found the colour you want to pick.
Tip: Your colours and palettes will be saved even when closing the Imagink application.