This is the dialog to allow you to curve in any colour mode - this is showing rgb, just after I have set the Neutral point.

Remember that moving the top end of the curve (the white end) left along the top axis and bottom end of the curve to the right, is the same as moving the 2 end sliders of Levels.
Moving the mid-point diagonally (to top left/bottom right) is equivalent (well almost) to the mid-point Level slider.

Notice that the individual rgb curves (just visible either side of the main diagonal) are shown along with the 'all' channel and also the histogram changes as you alter the curve, so you can see what is happening to the pixels.

Well worth playing with.