I want to neutralize a color using an inverse color, how do I select an inverse color?

Make a duplicate of the color on a layer just above and then use the Image/invert command and then set the layers transparency anywhere from 0% to 50%. 50% would be dead neautral but anything above 50% would start to give you the opposite color. You could also just use the desaturate command and desaturate to the desired amount.

Jerry

ps: If there is actually detail in the color (Like being an actual image) then also use the color blending mode for the layer containing the color you are trying to neautralize.