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
Metal is usually shiny and therefore should be white where light hits best. It reflects its surroundings, so many different colours could be used. Metal 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.
Darker shades can be replaced with different colours to create a higher contrast or to give the material an undertone.
The default shadow for wesnoth is purple. It works best on orange or brown shades, but can be used for almost every colour with good results.
Non-reflective surfaces (e.g. normal clothes) should have just darker shades of the base colour, grey or the default purple in the shadows. Metall and other reflective materials can use a larger variety.
Complementary colours (e.g dark green shadow for orange materials) let the surface shine a little brighter.
To create the illusion of reflection green (grass,forest) and blue (sky) shades on metall work best.
Other colours can be used to characterize a faction/unit (e.g. blue shadows for units with a water theme, red for a fire theme...)
Additionaly the number of shades you replace can have a large effect:
Replace only one shade for a hardly noticeable undertone. It's an easy effect with little efford.
Replace multiple shades with different colours to create properties for the material (e.g rusty surface, magical aura, unique (magical) metalls like mythril). It has to be carefully crafted, otherwise it will look very strange.
Replacing multiple shades with the same colour will change the colour of the material. It is only good when desired.
As general rules:
1.Replacing brighter shades will most of the time be more noticeable than replacing dark ones.
2.Replacement-shades should have (roughly) the same brightness as the original shade.
3.Replacement-shades for non-grey colours should have will be less noticeable if they have (roughly) the same saturation. Can be desired or not.
Highlights are bright shades with a different colour than the material. They will mostly be used for metalls, but can also give textile a shining ability (-> silk).
If you don't want to change the whole colour of the material, they should only be used very sparely (single pixels). Only use them as replacement for the brightest shades, otherwise use shadows (see above).
Theoretically you could use any colour (with high brightness) to highlight, but bright green and yellow work best most of the time. Like shadows, a hue with higher saturation will be more invading than a grey colour.