choose Coloring the entire image or the active layer

[T-buch]

Someone who knows the logic / systematics for when GIMP color the entire images (i.e. all layers) and when it is only the active layer? (with tools like "pencil"
For me it's a matter of luck. So once in a while I'm lucky: That is-  if I only want to color within an active layer, then it is also what happens
But.... I'd like to be able to be in control of the situation myself ;