What problem does this address?

At WordPress.com, we’ve implemented a Timeline Block that includes a default background color: https://github.com/Automattic/wp-calypso/blob/17256c5b7b410d332188f1a13c4b3055c1ea9de0/apps/editing-toolkit/editing-toolkit-plugin/jetpack-timeline/blocks/src/timeline-item.js#L135

A user can select a new background color for their Timeline Entry. However, when they deselect their color, they're returned to the default background color:

It's not possible for them to clear the default background color entirely. Ideally, they'd be able to select 'Transparent' as a color option.

What is your proposed solution?

While it's possible to achieve transparency with enableAlpha, it's not obvious.

I'd love some form of button that makes it one-click to apply transparent as a color.

Some relevant conversation in Automattic/wp-calypso#65266 (comment)