Heavily biased through the eyes of Abso's pumpkin mask.
This isn't a complex guide, just something you should maybe read :3
Might help your designs sell.
Or might make you mad at me.
Not saying you hafta follow this.
Theme
Some unnatural designs often go after a theme. Food themes are popular, especially for the closed species you see around deviantart. Some examples are weather themes, animal themes, etc.
Colors
Highlighter designs, or designs that literally look like they've been dipped in rainbow llama vomit, are quite often seen in Adoptables. Adoptables are breedable(usually) designs that are usually more simple can have two designs that are strikingly similar. WolfPlay, along with many similar SIM games should have atleast a few wolves that look the exact same :p I bet my life on that. Your designs on Wolfplay technically belong to the artist who made the markings. Kind of. The ones born and generated by birth anyways. I think lol don't trust me.
Highlighter designs are flashy. And although some people like them, it kinda takes the creativity away.
Usually.
<div align="left">
MarkingsDon't know about you. But I love markings. Unique ones. Ones that go with the theme and gives the characters something special to stand by.
Unique is not a squiggly line drawn randomly. Actually look at some markings of the thing you're theming after or something like so.
Depending on how well the colors go together, markings may or may not make a huge effect.
BUT.
DO NOT make bucket fill designs. Anyone can do that.
Bucket Fill Design: "A wolf with blue eyes and silver fur"
Unless it's an adopt or you're selling it for really cheap.
Buying Designs (Why?)
Why do we buy designs. That, I'm not sure.
In why me myself buy designs is mostly because the design just slapped me silly and made me go WOW.
I like it. I buy it.
Instinct.
But also because I'm too lazy to design something like that myself.
Fall 
Day Weather: 
