Shadowkin have tough skin and are covered in crystal/rock-like growths and spikes with different levels of pointyness, they don't have the need to use full armour, the growths will get in the way of most clothing anyway, so even armour of the shadowkin is aimed for dealing more damage, rather than protect, and the edgier it looks the better! They usually have very dark garments along with the details and spikes made out of metal found specifically on their territory, no one, however, stops them from adding a few dashes of favorite color here and there just to stand out a little.
Shadowkin would wear something more colorful if they deliberately want to be noticed, on celebratory occasions, or if they live on the surface and blending with the shadows is not a necessity (although, most still stick with the dark wear simply because they take pride in their style)