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)
Shading depending on material
Metall is usually shiny and therefore should be white where light hits best. It reflects its surroundings, so many different colours could be used. Metall surfaces usually use more shades than other materials
Skin and leather aren't (very) reflective. Oily skin is very shiny (-> high contrast), while dry or dirty skin has a lower difference between shades.
Textile isn't shiny or reflective at all. As result white can only be used for bright fabric. It also creates folds on its own, so light shades and shadows often alternate.
I'd generally advise against using the same shades in the same image on different surfaces as it can make later edits more difficult and look confusing, but sharing the palette between different units can be useful to indicate unity (eg. units from the same faction).
Nevertheless here are three units, who use only 12 shades of brown, to show you how to reduce colour count
These units create different materials and colours by using variating shading method (see above) and quantity of a colour.
The fist one has a lot of orange in his trousers to distinguish it from his skintone. His weapon uses browntones with a hint of purple and green to create the illusion of a natural wooden branch and beige for dry leaves. The golden ornaments are mostly yellow with white as higlights and orange/brown in the shadows.
Nr.2 has a darker but shiny skin, so the contrast is higher (white highlights to brown skin)
The last one has a lighter skin with mostly beige shades. On his shoulders are bronze plates with a lot more orange than the gold ornaments of Nr.1. His skirt and breastplate have nearly the same shades as the skin, but a lot more white (skirt) and brown (armor) to distinguish.
Most of the time the shape of an object can be decisive for the apperance of the material.