Color Settings include alpha channel for transparency

[fotisps]

a good feature for Gutenberg could be to add the alpha channel for transparency wherever (if it's possible) where the user can select a background or a foreground color since it is used a lot from designers.

The best way is to add it as rgba first or/and as hexadecimal with the 2 digits in the end like #00000020 (black with 20 as hex for opacity)