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Using Photoshop CS4 'Save for Web & Devices' -- I can tell with the Color Table how many colors are in use in the PNG-8, but am not sure how much downsampling to 256-colors is reducing the color depth of my image.

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    The only way to know is to try it and look at the results.
    – Mark Ransom
    Jun 15, 2011 at 19:24
  • Pretty subjective -- any tools out there that count colors in PNGs?
    – Craig
    Jun 15, 2011 at 22:27
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    Counting colors won't help, as it won't tell you if the colors dropped are similar to the starting colors or not. You can subtract the 8-bit result from the original, but again it's going to be a subjective measure.
    – Mark Ransom
    Jun 15, 2011 at 22:34
  • Try converting 24 to 8 and back to 24 then you can compare the results byte for byte.
    – Chris Nava
    Jun 21, 2011 at 20:08

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You can measure it with ImageMagick:

compare -mertic MSE original-input.png degraded-input.png difference-output.png

or with DSSIM:

dssim original-input.png degraded-input.png difference-output.png

MSE is mean square error and DSSIM is structural (dis)similarity metric. The latter is a bit more closely aligned with human perception of image degradation, but both are just estimates.

Also when you convert images to 256-colors with latest version of pngquant it will show MSE in verbose mode:

pngquant -v image.png
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