Is this a correct way of thinking about this?
IMO that is certainly a correct way of thinking about it, but let's get a little more detailed and specific:
If we want to take a stab at defining scale vs mode, we could say that those ordered collections of notes that have proven to be most useful as building blocks for our "modern" [call that post Renaissance, for argument's sake] music we call scales - for example "The Major Scale" "The Minor Scale", "The Melodic Minor Scale". ( @AlphonsoBalvenie in his answer refers to those as "Established Scales", a good working term IMO. ) Their alternate configurations, as will be explained - for example "Lydian Mode", "Dorian Mode", "Phrygian Mode" - those we call modes. However, that is clearly a subjective definition. An objective definition of scale vs mode differs significantly, as follows:
There is no objective difference between a mode and a scale: Take any key signature, play any ordered collection of notes using that key signature - start on any note in the chromatic octave (which has no accidentals) and play a sequence of notes abiding by that key signature - you now have what's called a scale/mode.
Mode is a relative term: when you decide, for whatever reason, to make one particular note of your ordered collection of notes the root - the first note of your scale according to the key signature - that configuration is called the scale, and becomes a 'parent scale', as it were, to its other modes: Building a scale with the same key signature but using a different root than the 'parent' root becomes a mode: A different fashion/manner of playing the parent scale.
So using the example of C Ionian - which we call the C Major Scale, the 6th mode of that scale - a scale that starts on the 6th degree of the parent scale, (whose key signature has no sharps and no flats) in this case A - becomes the Aeolian mode, commonly known as the A Minor Scale. But note that C major also has a modal name: Ionian. In fact, if we decide that A Aeolian is the parent scale, C Ionian would then be considered the third mode of A Minor. And so it goes with all modes and scales.
Because today we tend to think of the C Major and A Minor Scales as the basis for our tonal system, everything else we refer to as "modes", but it's by no means so clear-cut, and in truth it's simpler than one might think.
However, they can completely shift its tonality...
Correct. As you work through the different modes of a parent scale, the tonality constantly shifts - some modes have what we call major tonality, some minor tonality, and some tend to be a bit ambiguous as well - most notably dominant tonality, as will be explained. Generally how we classify tonality is determined by the 3rd and 7th scale degrees. Those distinctions are somewhat subjective, fuzzy and arbitrary, but are useful for discussion and analysis and can be helpful when developing harmonies or improvising over a mode or scale.
- Using the C Major scale (Objectively speaking, it's called the Ionian mode), the 3rd degree is E, a Major 3rd, and the
7th degree is B, a Major 7th - decidedly a Major tonality.
- Moving to the 2nd mode of C Major, built on D and referred to as
the Dorian mode, the 3rd degree is F, a Minor 3rd, and the 7th degree
is C, a Minor 7th - decidedly a Minor tonality.
- Moving to the 5th mode of C Major, built on G and referred to as the
Mixolydian mode, the 3rd degree is B, a Major 3rd, and the 7th degree
is F, a Minor 7th. This "split" or "ambigious" tonality, which creates a tritone - 3 'whole steps' or 6 chromatic tones and considered the most dissonant of intervals - between the 3rd and 7th degrees of the scale/mode - is called Dominant tonality. [Manipulating dominants in contrast to overtly major or minor scales and chords is idiomatic for creating a sense of tension and movement in the European/Western traditional musical system. Almost all rock, blues and jazz emphasizes dominants, which makes those genres both very flexible and often quite difficult to analyze according to the traditional rules of harmony.]
Aside from the tonality of the scale/mode with reference to itself, the diatonic chords, and therefore diatonic harmonies, also shift as modes change. For example:
- The diatonic 7th chord for the root note of C Major (C) is comprised
of C-E-G-B - a Major 7th chord: Both the 3rd and the 7th are major.
- The diatonic 7th chord for the root note of D Dorian (D) is comprised
of D-F-A-C - a Minor 7th chord. Both the 3rd and the 7th are
- The diatonic 7th chord for the root note of G Mixolydian (G) is
comprised of G-B-D-F - a chord that includes a major 3rd and a
minor 7th - creating a Dominant 7th chord (the default 7th chord when unqualified)
So, one could say that A Aeolian is the C major scale with an
emphasis on its 6th scale degree- Aeolian- known for being dark.
That's fair enough, but be cognizant of the fact that using C Major/Ionian mode as your reference point is based on the conventional western tonal system (as reflected in the piano keyboard - all white keys== C Major). Objectively speaking "all modes are created equal".
(As an aside, in modern jazz/pop parlance, the "default" minor scale is Dorian, not Aeolian. The m6 [F using A Aeolian] creates difficulties when building 'tasteful' chords and harmonies that are solved when using Dorian, which contains a M/Dorian 6th [B using D Dorian] - one might say Aeolian is a bit too dark.)