In a Chaos Army, there is no particular style that you should have in mind when painting the models. There is no order or method of applying the paint that will leave you with a better finish. You can paint your models however you want. The only thing you need to keep in mind regarding styles of painting is how quickly you want to finish your models, and how good you want them to look. However, it is a good idea to always be thinking about the colours you want to paint your models. Certain Chaos Powers need to use certain colours in order to make them fit in with the Powers background and style. For instance, a Chaos Lord of Khorne would not dream of being seen wearing pink and orange. The chart below shows my feelings as to what colours should be used with certain Chaos Powers
Khorne - The best colours to use on Khornate troops are obviously black and red. Bronze is another colour that fits in well with Khorne. Good colours to use for decorations such as plumes are dark greens such as Emerald Green.
Tzeentch - Tzeentch favours bright colours such as yellow, blue (a mixture of enchanted blue and regal blue looks good), purple and small amounts of pink. Always keep your Tzeentch warriors bright!
Nurgle - Nurgle troops look great in morbid brownish clothes. Browns, terracotta and brownish-greens are the best colours for Nurgle.
Slaanesh - Before the days of 'Warhammer Armies: Chaos', all Slaanesh troops wore pale colours. You can use almost any colour you want on Slaanesh, but it always looks good if you mix in small amounts of white.
I would also like to make the few points shown below.
Chaos Knights / Warriors - It often looks good if you paint chaos armour in the colours of their god. Remember that the armour is a gift from the Chaos Gods, and the colours used serve as an easily identifiable insignia. If you do decide to paint them in metal colours, make sure you feature enough of their gods colours on the model, otherwise it will look boring.
Daemons - Paint daemons using the colours of their god. This will make them look more like a follower of a particular Chaos God.
Standards - Try and use appropriate colours on the standards of your troops, so that they fit in with the colours you use on your models.
Beastmen - Many troops such as Beastmen do not have any thing on them that the appropriate colours can be painted on. In this case, you have two choices. Option one is to use their banners and shields to show which Chaos Power they belong to. Option two is to is a little more adventurous. You could try to decorate your models with warpaint that could have on the colours of whatever Chaos power the model belongs to.