Interactive color picker in one 9111-byte file (HTML/CSS/JavaScript)
Selected color is at clicked location in a square that is a cross-sectional slice of the color cube for your choice of RGB or HSL. One of the three channels (of your choice) is selected with a slider, and the other two are the coordinates of the selected point in the square.
An installed demo is at https://jsfiddle.net/n8chz/2fa0uymm/4/