Possibility to null color fields

[Meithal]

Following #5557

It is still impossible to null color fields.

And I dont see what the HTML standard has to do with the way SQL or PHP represent null values.

Or in a different approach why are you guys even considering using an html color field if it so limited; it doesnt even accept colors with an alpha channel.

[Meithal]

https://github.com/bgrins/spectrum