Come individuare la percentuale di un colore campionato in un'immagine?

You are both right. In fact I didn't read where talked about conversion pixel/percentage, sorry.

Many thanks for the quick help!

So to help you a bit here, I would say that the only way to record the percentage of any color is to know how that color is made and then use the proper sampeling method.

So you have 3 choices, because image colors in Photoshop are only in these three colorspaces.

1.) 16 bit Lab

2.) RGB  colour, but that RGB colour is based on a reference in Lab called an ICC profile and is dependent on that profile for the actual appearance of the color in RGB values.

3.) CMYK colour.  Basically the same problem as the RGB colours above.  The actual colour is dependent on the CMYK profile used to convert the Lab or RGB image. 

So if you actually need the exact colour values, use 16 bit Lab mode and record those values only. 

As you said you need a "percentage of sampled colour" then you may with to use a different method.

I would ask a clairifying question to understand the meaning of the statement above.  I understand you do not want pixel values, but what you do want is not exactly clear.   To understand what you need I would ask this: How are you defining "percentage of sampled Colors" ?  Does this meen where you place the cursor or does it mean all of the greens in this picture or something like that approach?

That will help this group provide an answer. 

Hi and thanks for your try to help.

I do not need the pixels, I need the percentage of a sampled color.

To help you to understand, imagine a square where half is black and half is white. The result is that in the square the black occupies the 50% of the picture. Now let's try to imagine a square with 4 colors: black, white, red, and blue.... what's the percentage of blue in the square? If all parts are the same it's easy, it would be 25% for each color. But how to determine the percentage when parts are differents in size? Also consider that parts in the reality are 8 (8 colors).

Hope it's finally clear, I really don't know how to better explain it.

I do not need pixels, I do not need RGB or CMYK values... I just want a percentage of presence of the color sampled in the picture.

Many thanks

Note the difference in pixel count. Convert to percent.
Click on image for larger view

Hello everyone I have to physically recreate the hue of a sand, or at least I have to get as close as possible. I took a close-up photo of a portion of sand and processing it on the computer I wish I could know in that photo how much color there is, as a percentage, for each color sampled with the eyedropper. Of course, I reduced the number of colors by also applying a mosaic filter to simplify the portioning: in practice I created colored cell macros. Absolutely I don't care about the percentages in RGB nor in CMYK but I need to know the percentage of the exact color sampled. I basically select the eyedropper, I go to the photo, I click on a color, and I want to know proportionately, compared to the whole picture (or selection, it's the same thing), what percentage is that specific color. Having the grain available in different shades I should then recreate the color by mixing the color portions according to the percentages detected. I hope I've been clear. With RGB or CMYK values I could not do it because first thing I do not need a homogeneous color and second thing I have no grain in CMYK or RGB but I have red, green, light green, dark green, yellow, black, white etc etc etc etc etc etc... How do I know the percentage of color "CAMPIONED" present in an image? Thank you so much for the invaluable help!

Hmm, the issue does not seem extremely clear I guess.

I wonder if the outlined process will really serve your needs.

If you want to determine the number of pixels of a Selection (ideally one without anti-aliasing etc.) you can check »Pixels« in the Histogram Panel (but refresh if possible!).

The percentage can then be calculated against the total number of pixels (width times height in pixels).

Instead of calculating that manually I expect one could automate the calculation with a Script, if it’s worth the trouble.