“Adoration of the Magi,” Botticelli
Using the brush tool in Photoshop,
sample a widely used color
in the painting you've selected.
Fill your box with the color.
I chose the lighter part of the sky in the background of the Botticelli above. Photoshop tells me this is Pantone 420 C.
Select a second color and fill a second box.
Stack it alongside the first box.
Don't worry about whether you've chosen the “right” colors. Just select the colors that feel right to you. Botticelli has already done the heavy lifting, so any of these colors will look good together. You can always come back to the painting later if you want to make changes.
This second color is the blue from the robes in the foreground. Pantone 432 C.
Continue this process until you have a pallette of 5 colors.
Don't forget to include a sample of a deep shade so you'll have a color for text. There are many shades that Pantone calls “black.” This is Pantone Black 2 C. In CMYK chromatography it's 60 C, 60 M, 80 Y, 83 K. Using additive color theory it's R51, G51, B51, also known as 3e3b28, or if you want web-only colors, #333333. Don't forget to click the Botticelli so you can continue down the rabbit hole. I want you to see my new hat.