Add coloring for completion of Color components

[Chaosus]

(Continuation of #74809)

[Calinou]

Tested locally (rebased on top of master 71699e0), it works as expected.

[Mickeon]

Using the axes colours makes sense, but feels somewhat "janky". Would it be worth to add other theme colours?

[Calinou]

Using the axes colours makes sense, but feels somewhat "janky". Would it be worth to add other theme colours?

We could have generic editor colors that can be reused, similar to what #77114 adds for its own needs.

[akien-mga]

Thanks!