I threw this together as a bit of an example to illustrate one main point.
In small rpg sprites where you only have 20-30 pixels height to render your detail, contrast becomes your vital ally. I tried not to stray too far from your color choice and character design and put together a little example to illustrate my point. This concept should work with or without a perimeter outline (that i decided to add) but the basics of light and shadow become much more interesting to portray in such small spaces.
remember that every pixel counts, and it will count more if there is greater contrast or change from one pixel to its neighbor pixel. subtle blends tend to fade or blur a character sprite at this size. This is one of the common threads youll find with the 'great' sprites from the older super nintendo Jrpg days where all their character sprites really pop.