Restructure ‘applied’ colour settings file

Better structure and document the applied colour settings file to make it clearer what each thing does.

src/globals/scss/settings/_colours-applied.scss

@@ -1,14 +1,47 @@
-// Semantic colour names
+// Text colour
+
+$govuk-text-colour: $govuk-black;
+
+// Text colour for print media
+//
+// Use 'true black' to avoid printers using colour ink to print body text
+
+$govuk-print-text-colour: #000000;
+
+// Secondary text colour
+//
+// Used for 'muted' text, help text, etc.
+
+$govuk-secondary-text-colour: $govuk-grey-1;
+
+// Links
+
 $govuk-link-colour: $govuk-blue;
-$govuk-link-active-colour: $govuk-light-blue;
-$govuk-link-hover-colour: $govuk-light-blue;
 $govuk-link-visited-colour: #4c2c92;
+$govuk-link-hover-colour: $govuk-light-blue;
+$govuk-link-active-colour: $govuk-light-blue;
+
+// Focus colour
+//
+// Used for outline (and background, where appropriate) when interactive
+// elements (links, form controls) have keyboard focus.
+
+$govuk-focus-colour: $govuk-yellow;
+
+// Error colour
+//
+// Used to highlight error messages and form controls in an error state
+
+$govuk-error-colour: $govuk-red;
+
+// Border colour
+//
+// Used for borders, separators, rules, keylines etc.
+
+$govuk-border-colour: $govuk-grey-2;
+
+// Buttons
+
 $govuk-button-colour: #00823b;
 $govuk-button-hover-colour: darken($govuk-button-colour, 5%);
 $govuk-button-colour-darken-15: darken($govuk-button-colour, 15%);
-$govuk-focus-colour: $govuk-yellow;
-$govuk-text-colour: $govuk-black;             // Standard text colour
-$govuk-print-text-colour: #000000;            // Text colour when printing
-$govuk-secondary-text-colour: $govuk-grey-1;  // Section headers, help text etc.
-$govuk-border-colour: $govuk-grey-2;          // Borders, seperators, rules, keylines etc.
-$govuk-error-colour: $govuk-red;              // Error text and border colour