I can see two possibilities that aren't related to post-processing either in or out of camera.

First, increase the light, either through changing the scene or by using wider aperture and longer shutter — and thereby lowering the required ISO. This doesn't seem exactly in the spirit of your question, though.

So, second: let the darker scenes be themselves. Our night vision depends more on cells in our eyes which don't distinguish between colors, which means that a decrease in saturation as the scene darkens is part of our natural visual language. Why fight that? You may even want to decrease saturation further in post-processing, which also works in your favor with low-light shooting, as chroma noise is usually regarded as uglier than luminosity noise. You might even consider going all monochrome.

That might be not quite what you wanted to hear either, so I'll sneak in a third: buy a better camera or wait til a few years as sensor technology continues to improve. Better high ISO capability continues to be in demand and will be a focus of research for the foreseeable future.