How to create a curved color spectrum?

[Contours]

I'm trying to create a curved "spectrum" of color to use behind the numbers on a gauge.

I first tried a linear fill, going from green to red. It's easy, but it doesn't produce the yellow in the color transition.

Next, I tried placing swatches of colors and then using the arch as a mask ... but it would only pick up the solid colors, so there were no transitions.

Does anyone know of a method that might work? Is there a way to bend the staight color spectrum bitmap shown in the graphic?

Thanks in advance for any help.