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It does one no good to specify a color for print output that your printer cannot produce. It’s also counterproductive to use cached Histogram data to make certain technical decisions about image purposing. PS identifies and abbreviates the resolution for such problems.

In this case, let’s presume that you are going to draw red outline around a particular element in the example aerial below. You’ve already set your View>Show All Menus>Proof Setup>Custom to your intended output profile (meaning that yes, this really should be customized for each output device in the workflow), and it (your intended output profile) is checkmarked at the bottom of your View>Show All Menus>Proof Setup drop-down menu.

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I have selected a Red that violates my output Gamut. Photoshop is alerting me to that fact by displaying a little warning sign (indicated by my bright Green, print-Gamut violating arrow) and displaying the actual color value that will result when my image is remapped to said output Gamut using the Rendering Intent specified in my Edit>Color Settings>Conversion Options Dialogue Box.

Photoshop allows me to “correct” my color selection by simply clicking the warning icon.

Extremely sharp readers will note that the Hue input in the HSB Color Values allow Hue to be specified by degrees on the Standard Color Wheel. You’re welcome.

There’s another such icon that appears in your Histogram Window when the Histogram is displaying cached data and not the actual (“real time”) Histogram data. I think Adobe should change this particular icon to a comb icon (inside joke…note the profile of the Histogram):

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Clicking anywhere within in the Histogram Window refreshes the Histogram data, but keep in mind that Photoshop is “guessing” at the colors its repopulating in between all of those spikes (which represent the data from the image’s previous state). This particular “Resampling” is not of the same genus as the interpolation methodologies you are familiar with for spatial adjustments, e.g. Nearest Neighbor or Bicubic-I don’t know what algorithms Photoshop uses for this application. It may just average!

In other Histogram Views (e.g. Expanded) there are additional controls for refreshing your Histogram display.

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