Exercise 7.1 The Image: Colors > Threshold tool is difficult to use well, and, unfortunately, it generates a black and white image. But you can choose a subrange that preserves the most significant part of the image. And you can often get interesting results by choosing a short subrange to produce something similar to an inverted etching.
Exercise 7.2 Screen mode is a good candidate when combined with an opacity reduced to 40% or less. Normal mode with a very reduced opacity can also be handy.