Choosing your palette
When choosing colours for your sprite art it is usually best to use already existing (mainline or UMC) palettes. But what can you do when you want to create your own, individual units?
I'll try to give you a short lesson -and hopefully some insights- into colour theory.
Analysing existing art
Some of the newer mainline sprites use a lot of shades you (at first) wouldn't expect. Before creating your own images it is good to look at and analyse them. Good examples are the Firedragon(green and purple shadows) and Elvish Outrider(yellow highlights on metall) Those colours, sparingly used, can create contrast in the picture without breaking the whole image.
Choosing number of colours
Any material (which doesn't lie completely in shadows) should have at least 4 shades. Bigger surfaces should have up to 6, while the most dominant can have around 8 shades, but try to keep the number low. The shades you need are:
1. White: for shiny or bright surfaces
2. Highlighting colour (optional)
3. Base-colour of the material in normal light
4. Blackish outline (Don't use black; The outline can be the same colour for the whole unit)
5.-8. About 2-4 Darker shades of the base-colour (can be replaced with other colours; more of that later)
Same colours for different surfaces
The first lesson to learn is, the same colours can be used for various different surfaces.
Not only does this fairy use the same colours for her dress and wings as the outrider uses for his metall armor, but some hues are also shared between dress,hair,skin and staff.
I'd generally advise against using the same shades in the same image on different surfaces as it can make later edits more difficult, but sharing the palette between different units can be useful to indicate unity (eg. units from the same faction).