Layer Masks are very helpful and learning how to use them will definitely save time and work. Tstandiford was right about the eraser to remove the actual top just to show the layer/s underneath it. It doesn't help when coloring and reverting the regions that was erased unlike masks that allows change from transparent (invisible), opaque (visible) or semitransparent (visible but can see through) represented by black, white and grey without destroying the image. Some things I do is duplicate the original and then make sure the layer mask is selected correctly (highlighted) so as not to paint anything on the duplicate.
Using gradients as layer mask is also better than what I do (using selections of blurred shapes then paint in mask). Some things to note: the mask and its layer share the same opacity % and colors will turn into greyscale once placed in the mask. Thanks for an introduction of "add Greyscale copy of layer" because it is new to me (looks like the negatives of camera films). It will be useful for my future Gimping.
Layer masks also works with layers with modes other than normal. They are also used in tricks to emphasize a subject with a colored layer with mask on top while its greyscaled one is below.
This is a great video for Gimpers who are not yet familiar with layer masks. Straightforward, simple, highly detailed and very easy to understand. Thank you again for this video (will watch your other tuts).