Inconsistent way to customize color in different places

Problems

Users can assign custom color by entering hex color code on new label page and in the label dropdown menu in the issuable sidebar. However, users only can pick up a color through the default "Colors" window on MacOS on /admin/broadcast_messages.

It's inconsistent with the ways on GitLab.com, and the default way on MacOS is also not very sufficient since it's hard to pick up an accurate color.