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Is this style against the rules?

(+1)

I think so, though I'm not the qualified judge on that I can see depth in it as you've shaded the penguin to have a light source in front of it. So I'm pretty sure it's not allowed.

I actually just wanted it to be coloured differently, because a pure black shape will be hard to differentiate parts. I can see it looks like shading, but for me it was a stylzed outline.

(+1)

The front area looks like it has been 'burnt', which is a form of shading.  I have no issues of using a darker color, like black, to differentiate between different body parts. 'Using  'dodge' is also not advised.  It too is a form of shading.