This is a simple color picker realized in React

The picker roughly looks like this:

You can choose a base color on the layer panel first and then range and/or alpha at bottom.

You can also define your color by typing.

If you're trying to figure out the principle of the web color model, you may refer to:

Web colors - Wikipedia

RGBA color space - Wikipedia

usage

npm install simple-react-color-picker

Rendering:

import React from 'react'
import ReactDOM from 'react-dom'
import PickerPanel from 'simple-react-color-picker'

...

render(){
	return(
		<div>
			...
 			<PickerPanel />
 		</div>
	)
}