I use the color picker, but not necesarily for exact numerical rgb values.. If I am having trouble matching a color ill use the picker to select a good avereage and fill a large area to give myself a visual aid. If i am using a photo reference, ill make a document at the size of my canvas, for example 10 X 17 @ 300Dpi and frame my reference photo in there as i like it. Then i bring down guides usually at 1 inch incriments ( you can also view > show> grid but i like the colored guides better). I then blue-line the same grid onto my actual canvas. Now I can use my screen as reference, even zooming in and out as needed, to draw my sketch onto the canvas. Copying a black and white layer helps here as well. This B/W layer can also be used to aid underpainting, if that is what i'm doing, then color turned back on for color. Ive found Photoshop can be a powerful tool, even if its not being used to create the final piece.
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