Challenge 1: Selecting a Mostly-Uniform Sky — Solution Walkthrough

The tool to reach for first:

Magic Wand (Photoshop) or Select by Color/Fuzzy Select (GIMP) — a
color/tone-based selection tool is the natural fit for a large area
that's mostly one continuous shade, per this chapter's own description
of what these tools are built for.

The setting to adjust if some patches are missed:

The tolerance setting. Per this chapter's own material, tolerance
controls how similar adjacent pixels need to be before they're
included in the selection. If a few slightly different-toned patches
of sky (near clouds, or subtly different shading) were missed on the
first attempt, increasing the tolerance value would allow the tool to
include a wider range of similar-but-not-identical blue tones in the
selection.

Why this is the right first adjustment rather than switching tools:

Since the core problem (some genuinely sky-colored pixels weren't
included) is directly about how strictly "similar enough" is being
judged, tolerance is exactly the setting built to address that —
there's no need to switch to a different selection tool or method
first when a simple tolerance adjustment addresses the actual issue
directly.

WHY THIS WORKS AS AN ANSWER
------------------------------
This exercise is a direct, practical application of this chapter's own
Magic Wand/tolerance material to a realistic photo-editing scenario,
correctly identifying both the appropriate tool and the specific
setting that resolves the described problem.