On iPhone, select the colour icon in the toolbar to switch the bar to the colours and palette mode.
On iPad, the colour menu is accessible from the right panel, in the brush section.
To change the colour of your current brush, select any colour icon in your palette. To modify a saved one, click on it twice to open the colour selector. You have three palettes available, just swipe up and down to select the one you want to use.
Tip: Your colours and palettes will be saved even when closing the Imagink application.