What tool can I use to fade between two existing colors in an image?

There are a number of ways to do that, but I feel that you're fixing the wrong problem, because there are other ways to remove the unwanted streak of color (content aware fill, generative fill, frequency separation using the mixer brush, perhaps even the patch tool or healing brush), that would do a better job than the clone stamp.

That said, one fairly simple way to tackle what you have now would be to copy a section of the column immediately below the dark patch onto a new layer, make it a Smart Object and set the blend mode to Lighter Color. Apply enough Guassian Blur to get a nice blend, then mask off what you don't need. Finally, add back enough noise to match the grain.

Another might be to make the dark and light into your foreground and background colors, add a layer, make a selection around the area where you want to smooth out the color, fill with the gradient and experiment from there to get good-looking result. You'll then want to add noise, as before.

If you're up for some experimenting and you like to be hands on rather than leaving it all to an automated tool, definitely look into frequency separation retouching for this. There were three sessions at MAX 2020 covering the technique in depth. The link I gave is to the first one, from where you can find the rest.