Okay

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I just need tips and suggestions on how to improve this.

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You mean with markings?

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No, the base itself. I'm afraid people will say it looks too much like slate or obsidian.

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I don't think it really looks like either.
It's more of a middle of the road of the two.
I tried to find it in wardrobe and was getting frustrated. It looks nice!

Maybe do some light around the eyes or inner ear?

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The mane looks good! It's different enough from Slate to be immediately noticable. The base itself /does/ look quite similar to Slate though, other than additional mild pale shading on the toes, belly and muzzle. The countershading could be deepened a tad more, maybe? Coal is dark, very very dark so some deepened shading or even pock marks could help differentiate it more.

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Made some edits, but now it seems more like ebony. ;-;

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Its not really brown tinted like ebony though. I like the second to last edit XD

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There are options, definitely options.
It reminds me of Ebony because of the purple tint.

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Second to last is the best one so far imo, looks really nice. Countershaded? Blend the light parts and dark parts a tad bit more maybe? ^-^

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