Slime is a fun but tricky substance to paint, because it's highly glossy and somewhat translucent. It's neither quite solid nor liquid. If it's dripping or flowing down then you'll be able to see faint traces of object shapes and light sources behind it. Regarding light interaction, slime behaves very similarly to stained glass.
It's generally considered greenish-yellow, but you could apply these principles to any colour, and since it's very glossy it will also have very sharp highlights.
I have my character already composed here, and I know that I want the slime to trail from his nose to the totally inadequate handkerchief. My approach is to lay down a basic diffuse colour and then let the light come through where you'd expect it to, applying shadows and background shapes accordingly.
Then it's just a matter of adding those glossy highlights and reflections. I've drawn the goblin in a coloured, line-art style using Photoshop, so I also want to make sure the slime matches the background when it's done.
01. Start your slime trail
I create a new layer and use Photoshop's Pen tool to draw a selection path that will become the outline of the slime trail. I fill this selection with a slimy green colour, which I use as my diffuse colour base. Then I lock the transparency of the layer.
02. Add shadow for translucency
I brush in dark values where there's less light, sampling from the background and painting in some of that light colour. I'm suggesting translucency by lightly painting in what's behind it. I avoid using an airbrush because I like having painterly edges in my art.
03. Highlight to add depth
To reflect the main light source, which is from the top left, I add crisp, bright point highlights. Then to match my line-art style I outline the shape of the slime trail. To further suggest volume I add rim light reflections, from the secondary light source to the right.
This article originally appeared in ImagineFX magazine.
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