Adjusting colour in highlights or shadows only?

I'd try Warmth first. The temperature of the light can have subtle effects.

But the thing is, all the adjustments interact, really. For instance: "Contrast" increases the difference between light and dark, pulling them apart, "Shadows" shoves dark toward light-- but that means it also changes contrast. But it affects a different part of the dark, has a different center and range, so you can adjust both to get a more subtle effect. "Black Point" adjusts the darks like Shadows, but again, differently. Highlights changes contrast from the other end. Brilliance adjusts contrast in different parts differently, and sometimes it can do some cool things.

If the colors (American Spellcheck…) aren't all the same (if it's not all gray,) then some colors may get adjusted differently than others, and that may be what you're needing. Adjusting Black Point, Contrast, Shadows, Brilliance, and the levels curve may change colors unexpectedly. Selective Color may reduce blues everywhere, but you may be able to recover some with the other adjustments.

So there's no simple answer. If you want really subtle changes, you may need another editor. Some editors, like Lightroom, for instance, let you separate the shadow so you can change the color there independently of the other areas. Also, other editors have Shadows and Contrast, too, but they do slightly different things. I use both Lightroom and Photos, and I can do different things with each. But Photos is pretty powerful, and Lightroom costs money.