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.
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).
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 surfaces (mostly metall, but can be used for hair and skin aswell; Don't use it on non-white cloth)
2. Highlighting colour (optional; it is a bright colour and can be used to give a material a special shine; it can be a different colour than the material e.g. yellow highlight on the outrider)
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. Darker shades of the base-colour; Some can be replaced with other colours without changing the aperance of the sprite.
Replacing the shades
But what colours look good? Let's take a look at these armors.
The first one uses the colours of the loyalists and therefore would blend in best with other loyalists.
Nr. 2 and 3 just use grey; one a little brighter shades than the other
The interesting ones are in the lower row. The first uses a mixture of different colours (from dark to bright:green,brown,blue), that interact with each other. As result the aromor still looks greyish, but now has a bit of a green undertone.
The second just replaced grey with brown and therefore completely changed colour.
The third also uses brown and a blue/green tone (like the first one), but here the brown is the darkest colour and therefore the blueish looks more noticeable
In the last one I used gold, but replaced used some red shades instead of orange.
The replacement colour should have a similar brightness as the original colour.