Hello,
thank you for the kind words. A default colour and opacity of your own for free drawing cannot be saved at the moment. The colour, line width and opacity you choose are used for the following strokes as long as the editor is open, but the next time it starts again with the default values.
In the online editor you can set them before drawing: choose "Freehand" and then the button "Color and line width": https://tools.pdf24.org/en/annotate-pdf
Thank you for the suggestion, we will add it to our list.