How to make a Color Picking GUI? (Color Wheel)

They most likely would've used the Mouse position in the GUI's 'local space' to get the value it could be, meaning the actual image itself has no point in the actual code.

For example:

local Mouse = game.Players.LocalPlayer:GetMouse()
-->> Other variables just for example

local function GetRelativePosition(Object,MousePosition)
	local X = MousePosition.X-Object.AbsolutePosition.X
	local Y = MousePosition.Y-Object.AbsolutePosition.Y

	return Vector2.new(X,Y)
end

local function GetColor()
	local RelativePosition = GetRelativePosition(ColorPickerFrame,Mouse.Position)
	local ScaleX = (1/ColorPickerFrame.AbsoluteSize.X)*RelativePosition.X
	-->> Say it was 100 pixels in size, and the relative position was 50 pixels, the scale would be 0.5

	local Color = Color3.new(ScaleX,ScaleX,ScaleX)
	-->> Would go from black (scale 0) to white (scale 1) based on position
end

It's a bit unexplained, but I have to go, sorry.

Hope I helped!

~TDP