Just to touch back on Skin appearance with the goblins (and more generally).
Our closest example being Human skin. It has very interesting properties, it is semi translucent, a sort of waxy light-spread mechanism. The blood acts as a backlayer of red, the front a sort of tint of white. One of the most important mechanics is how it looks at different angles and the cue of the varying densities. Pigments can make a significant difference too. Even down to the diets, famous saying "We are what we eat", it reflects back onto how healthy our skin is, although interestingly enough there are others things as well, other creatures might attempt to 'fake it' to cope with deficiencies to have better survival/mating opportunities.
What I am trying to say is that skin isn't just a simple solid colour laid over the flesh and bone. There can be just so many mechanics and properties involved. Also we can introduce even unique effects not seen before in nature. Carpaints. Carpaints are interesting, flakes suspended inside a translucent surface ontop of a solid layer, to give amazing effects. Look at a surface straight on, it is dark red. Look towards the edge, it becomes a bright blue, an amazing and contradicting experience. You can even have flakes that glitter at specific angles of incidence.
It is far from being limited to skin btw. I think it would be.. very cool if the goblins had patterns, spots and marks to reflect on personalities, behaviours and such. The spots might accumulate near joints and such. Oh and hair/fur too are interesting, in RL, dogs will naturally go for natural oils, such as from fish, it makes their coats shiny (this might be similiar to carpaint flakes because oils might only grow on the hairs for a short period (Say a short period of successful predation), the shiny bits in the hairs might be only visible at certain angles due to coverage of other hairs). In DF we could reflect this back on predators, a named beast could have a shiny coating and marks to reflect on its' kills.
Biomes, having a tendency to affect creatures over long periods, especially with Spheres. An arid desert could have more effects than just culturally, but also physically in appearance. For example an Oasis lake being the only source of water is tainted with certain natural blue minerals and magical red elements. Creatures drinking from the Oasis might slowly and only slightly turn with a hint of midnight blue with dark red flakes in sunlight, but at night with a glowing cue of vicious red along the edge of the creatures.