Are you still trying to color your knobs???? You can use a D knob on a spline, by drilling the knob out one drill size smaller than the spline on a drill press (calipers might be needed for this one) Its called an interference fit and if you know the precise measurement of the shaft diameter you can probably find the correct size drill online.
I have made a D-knob out of a knob with a spline shaft before by taking a piece of metal shim stock and trimming it. I just stuck it on there between the splines, but I'm sure once you knew which splines positioned the shim stock just right, you could epoxy the shim stock to the knob for a more permanent solution.