You can use the http://yanrishatum.ru/pj/?input=http%3A%2F%2Fpixeljoint.com%2Ffiles%2Ficons%2Ffull%2F0x__r13181645221.png - PJ Image Specs tool to detect the infrequently-used pixels, though you'll have to replace them by hand in your source file.
For the gradients in the eyes, I think the simplest option would be to pick a few colours you like, and then draw the gradient by hand as a series of concentric circles using those colours, instead of trying to make a smooth gradient with a large number of colours. Those eye-gradients in particular look out of place since the rest of the piece has a low-colour-count approach.
The horns look like they were perhaps resized automatically, as they seem to have some blurring and a sharpening halo around them. I think going over them by hand to correct the halo and replace the AA with a few carefully chosen colours would make them look better.
Although this isn't an acceptance issue, you could improve the overall appearance of this piece by adding some hue-shifting, changing the hue of your colour ramps as they get darker/lighter. For example, when fading from bright red to black, don't just use a bunch of progressively darker reds, go from red to darkish red-purple to dark purple to black. When shading greys, it's particularly helpful to introduce some hues in there, since greys tend to look boring.
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