Flarum is using the native HTML5 color picker. Which I believe is the most sensible choice for letting users select a color.
There could be a javascript picker as a fallback if the native isn't supported, but that wouldn't change anything to this issue as the issue is with a native picker.
The way the user will actually pick the color is defined by the web browser. So if you're not happy with the picker, you should probably discuss that with the makers of the browser you use (or change web browser ? )
That being said I believe most browsers (well at least Firefox) use the system color picker. So now it becomes an operating system issue ?
For example I know on Firefox+Windows I have the same picker as you, but on Firefox+Ubuntu I get the following picker:
Also here's how the picker apparently looks on Mac (I took that image from MDN)
And of course on mobile you will probably have yet another look (if it's supported at all).
So well... Native inputs for the win ? ?
