Paths & Masks

Paths are a good way to  select,  copy, make designs with precision.
you can turn selections to paths and also paths to selections. They can be copied, duplicated, moved, transformed etc Right click on the path in the dialogue and you will see the menu.

Paths can be visible or invisible. Like layers they also have 'eyes' next to them when you are looking in the specific path dialogue. 

An edited path

 Layer  Masks are are non destructive way of changing an image.

After starting with an image I duplicated the layer (click on layer up the top and down to duplicate) and desaturated that layer, then added a 'black/full transparency' mask to the top layer. Then using a white paint brush I painted over the butterfly to make it all black and white. Taking all the extra colour out of it. If you go outside of the area you wish to change colour you can use the other colour -  black

Adding a mask to selection - you can only change things inside of the selection then unless you invert your selection - then outside will be your workable area.

If you use 'Quick mask' you will be left with a selection around the area you paint
Quick mask can be found at the bottom of your Gimp on the left hand side of the image window.

This will cover the image with a transparent colour when  active. It is red by default but the colour can be changed in preferences.

You can also use black and white images to quickly change something. 

Usually with masks a white paint brush will keep what is under it and a black mask will make it transparent.

When you have finished doing what you need to do with the mask except with Quick mask (Just click the corner icon again)- right click on the mask then on apply mask. It will then disappear. If at this stage you can see you need to make a change (In this session) you can undo apply  (edit > undo).