Import histogram

Have you tried the Filter > Neural Filters > Color Transfer?

OK that works to highlight these parts. But how can I do this per channel? I dont see any obvious way to do so. And then bonus question when I higlight red 255 for instance how can I bring those to 250 for instance. 

Apologies if my questions are super naive and mis formulated...

By @Daniel Roy

No problem…Asking questions is why this community exists.

The Levels and Curves adjustment layers let you offset levels by channel. They also offer clipping previews. The demo below shows one way to set level 255 to 250 in the Red channel.

The steps shown in the demo are:

1. Add a Curves adjustment layer. I did it through the Adjustments panel, but you can also do it through Layer > New Adjustment Layer > Curves, or by clicking the Create New Fill or Adjustment Layer button at the bottom of the Layers panel. 

2. In the Properties panel for the Curves adjustment layer, set it to the channel you want (or composite view for all channels).

3. If you want to preview current levels using a threshold view, hold down the Alt key on Windows or the Option key on macOS as you drag the white point or black point triangle at the bottom of the Curves graph. The reason it shows red when I do it is because this is in the Red channel.

3. Select the curve point you want to edit. You want to adjust level 255; that is the point at the end of the curve. When a point is selected, its Input (current) and Output (your change) values are shown below the graph. Note that for reference, the histogram is shown behind the curve.

3. Change the Output value of that point to whatever you want. You wanted to set it to 250. You can do that by dragging the point or editing the selected point’s Output value below the graph. Pressing any arrow key nudges the point by one level.

Even though you may not have known how to phrase the question, if this is what you were after it was a good question, because adjusting levels this way with Curves or Levels has been a fundamental Photoshop image correction technique and marketable skill for over 30 years, and therefore well worth asking about. To adjust tones between 0 and 255, you click on the curve to add a point at the Input level you want, and then you adjust that point’s Output value. In this way, you have control over up to 255 levels.