Document a way to add a apply button

[kantum]

What problem is this solving

Depending on the design requirement, it might be a good idea to add an apply and cancel control on the color picker.

There is no documentation on how to do this.

Proposed solution

Is there someone that knows how to do it? We may want this in the doc.

Describe alternatives you've considered

Looked for others colour picker libraries, didn't find my happiness.