[Stretch] Refine markup on the Colour page

[kellylee-gds]

What

Tidy up the markup to convert our swatches into dl's and re-style them.

Why

So it's consistent with the markup used in the Brand Guidelines.

Who needs to work on this

@owenatgov @CharlotteDowns @calvin-lau-sig7

Who needs to review this

TBC

Done when

TBD