I think this would be great!
You could imagine that, below the special (RGB) and (HSB) colors in the color keyboard, there could be a dotted outline around a blank “custom” color, and just like you can add Object or Game variables, you could add a new Game color that you specify with an HSB color picker. If it’s well-designed, you don’t have to know or care what the actual HSB coordinates are, you just appreciate being able to pick off a color from a color disc (and brightness slider) without having to switch to an entirely different HSB color picker project, and write down HSB coords that you find there. Or, maybe if you long-pressed on one of the pre-defined colors, it could bring up a color picker to change what that color is.
We like being to drag objects around in the main editor window to set their position, and pinch to set their size, without needing to always write out the position coordinates or size percent; I don’t see why the same visual interactive convenience shouldn’t extend to picking out a color.