Since you seem to be going about the visual design of the OC first, I think Pony Creator is a good start. It doesn't take very much effort, but it's good at showing off what your character looks like even if you're not good at drawing. I don't know the common opinion on it, but I think you should use it if you want to execute what the character looks like.

In terms of designing as aesthetic for your OC, I have a few pointers. 

• First, you want to identify your OC's personality/talent.  For example, if it's a Christmas themed OC, you'd know then to use Christmas colors, such as white, light blue, or green and red, etc. 
• Second, choose colors that match.  A good number of people, even those with little knowledge of color theory, like myself, will easily be able to spot a clashing color palette.  If you're not good at matching colors, then I'd talk to someone who is.
• Third, avoid using neon/oversaturated colors as your primary color.  When used in small amounts, such as for minor details, they aren't as terrible as most people would make them out to be.  It's only when your character's entire body is neon green that it becomes painful just to look at them.
• Finally, avoid using rainbow color palettes.  This is mostly a problem within the MLP community, since you have Rainbow Dash establishing the precedent.  I can understand that you might want to make your OC stand out, but it's also important to note that people are most likely to appreciate an OC that looks original.  Plus, on the vast majority of OCs, rainbow manes just look garish.