Replacing a brand or palette color by eye can leave stray pixels behind, especially when the source file mixes flat areas with antialiased edges. ImageMagick can target a specific color value and write a new image so the original file stays available for comparison.

The -opaque operator changes pixels that match the source color, and the -fill setting supplies the replacement color. Place -fill before -opaque so the operator uses the intended replacement value.

The examples below use ImageMagick 7 with the magick command and hexadecimal sRGB colors. Use a lossless output format such as PNG when the replacement must remain exact, and add a small -fuzz value only when compressed or antialiased pixels are close to the target color but not identical.

Related: Create a transparent background with ImageMagick
Related: Compare images with ImageMagick

Steps to replace a color with ImageMagick:

  1. List the colors in the source image when the exact value is unknown. 
    $ magick palette.png -format %c histogram:info:-
        108000: (34,197,94) #22C55E srgb(34,197,94)
        108000: (239,68,68) #EF4444 srgb(239,68,68)

    The histogram output is easiest to use on logos, icons, sprites, and other images with a small palette. Large photos can contain thousands of colors.

  2. Replace the exact source color and write a new output file. 
    $ magick palette.png -fill '#2563EB' -opaque '#EF4444' palette-updated.png

    In this example, #EF4444 is the color being replaced and #2563EB is the new color. Quote hexadecimal colors so the shell does not treat # as the start of a comment.

  3. Use -fuzz when near-matching edge pixels need to change with the same color. 
    $ magick palette.png -fuzz 8% -fill '#2563EB' -opaque '#EF4444' palette-updated.png

    A high -fuzz value can replace colors that only look similar to the intended target. Start with a small percentage and inspect the output image before replacing a production asset.

  4. Confirm that the old color is gone from the output palette. 
    $ magick palette-updated.png -format %c histogram:info:-
        108000: (34,197,94) #22C55E srgb(34,197,94)
        108000: (37,99,235) #2563EB srgb(37,99,235)

    The updated histogram should show the replacement color and no remaining row for the original target color.

  5. Open the updated image and compare the changed areas visually before using it in the final project.

    If a distribution only provides ImageMagick 6 command names, use convert with the same options instead of magick.