Image Color Palette Extractor
Quantises the image down to eight dominant colours and lists them with hex values.
Palette
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Swatches
Worked example
The percentages are shares of sampled pixels, not of visual importance. A large area of sky will dominate the list even if the photograph is about the subject.
Palette extraction counts pixels in colour buckets and returns the largest. That is why the result often looks duller than the image feels: the dominant colour of a landscape photograph is usually sky or foliage, not the thing you noticed.
Getting a useful palette
Crop to the region whose colour you want before uploading. A tight crop of a subject gives a palette about that subject; the full frame gives a palette about the scene.