FR: Option to limit native Colour field to presets only

[wearereeds]

Have an option on the colour field to limit it to only presets, e.g disable custom colour inputs / selection so the text field is readonly and disable/hide the other option on the preset dropdown.

Would be useful in scenarios were we want to limit colour options clients are allowed to select e.g. brand colours etc and would be nice for this functionality to be native rather than having to use a plugin.

[brandonkelly]

Got around to this for Craft 5.6! (#16249)