When opening the program you get to see thumbnails of the available pictures. Those have to be in the subfolder „Images“ and the masks/the mask texts have to be situated in the folder „Masks“.

Double click on the chosen picture.

The paint mode opens.

Here you can colorize the masked image areas.
The image areas can be selected directly on the picture or in the masks list.

Choose a RAL colour.

If the colour is to light or dark you have to move the lightness control to another value.