Fairly often in Photoshop, I liked to twiddle with some colour adjustment dialogue box (Levels, Curves etc.) and see instrumented on the sidebar both the present and adjusted numerical RGB values for whatever pixel is beneath the pointer without having to commit the adjustment. This is so that I could interactively explore the strength of the adjustment at various points of interest or match one part of a scene being adjusted to another part (outside my selection or in a different layer).

If a pixel-under-pointer display normally appears as

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R: 230
G:  18
B: 110

A suitable before-after pixel-under-pointer display, activated during an interactive adjustment, might look like

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R: 230 → 255
G:  18 →   0
B: 110 → 102

depending on the pixel format being used.

This simple docker display could be augmented by a settable swatch which causes RGB contour lines and highlighted areas to appear on the original or preview image wherever pixels match or cross the threshold set by that swatch. Programming such a thing might be quite involved, though, and much can be done with a simple before-after pixel-under-pointer display.