All menus contain items for changing colours.
Selecting one of these items displays the colour editor. In the upper left of the colour editor are a number of different coloured boxes. You can select one of these colours just by clicking in the corresponding box with the left mouse button. The colour you want to change in the picture changes, too, but this is not a permanent change until you click in the box labelled OK which also shows the current selected colour. If you have set a colour with the background or foreground option in the viewmolrc file, the rightmost coloured box shows this colour and you can apply it to your picture simple by clicking on it.