Features of Light and Darkness has a simple editor that you can use to create and save new light sources and pigments of your own. To get into the editor, touch any button that opens a selection list for either light sources or pigments, go to the top of the list, and press the "+" button at top right. The editor will open with a pre-loaded light source for you to work on, but you can use the "Load Light Source" or "Load Pigment" button to load something else.
The way you edit the light source or pigment that is loaded, is by pushing on it. Touch anywhere close to the spectrum -- anywhere out to the boundary of the yellow area will do -- and the spectrum shown will move up or down away from the point you are touching. The closer your touch is to the line that defines the shape of the spectrum, the smaller the area that will be affected, and the faster the change will take place. There is a "Smooth" button that will help smooth out any jagged edges you may inadvertently introduce, and there are "Undo" and "Redo" buttons that work the way you would expect. A "Start Over" button takes you back to the way things were when the editor first opened.
When you are happy with your changes, use the "Save ..." or "Save Copy ..." buttons to put your work back into the lists of light sources or pigments where you can use it. You cannot overwrite any of the light sources or pigments that are built in to Features of Light and Darkness -- the "Save ..." button will not work for them -- but you can create a modified version of any of them and use "Save Copy ..." to give it another name.
You can delete any light source or pigment that you have created -- but not the built-in ones. You do not delete these items from within the editor. Instead, open any of the lists that show light sources or pigments, and make the same one-finger brush-from-side-to-side gesture that you might use to delete a message from the list in the Mail app. Remember, though, that won't work for the built-in light sources and pigments.