This section gives general advice on painting techniques for terrain - instructions for painting specific terrain pieces are also included in each project page.
I use three types of paint - acrylic modelling paints (usually Citadel paints by Games Workshop), ordinary poster or powder paints (from art materials shops or toy shops) and household emulsion and oil-based paints. I use the household paints for covering very large areas such as terrain boards, and textured household paints for buildings that want a textured but no too-rough finish (textured paints are sold to mimic various stone finished and have a fine sandy material added to them).
For smaller jobs that still need a lot of paint I might use poster or powder paints which come in big squeezy bottles or tubs in the case of powder. Powder paint is especially cheap and you can mix it up into a dryish paste which drybrushes quite well. These paints are good for hills or large buildings where model paints would be expensive because of the area to be covered.
For small pieces and for fine details and finishing on all models I use acrylic model paints, usually Citadel. For plastic surfaces some kind of model paint is essential because the poster, powder and household paints don't really take on plastic finishes.
The basic approach to painting terrain is to get a base coat of the appropriate colour and then use drybrushing and washes to bring out the detail and give a more realistic effect.
For 40K buildings that require a darkish look I usually start by spraying the whole thing black and then building up the colours to grey or brown with successively lighter shades. I recommend black as a starting base for all but the lightest, most brightly coloured buildings because most buildings are very angular and black accentuates this to give the effect of deep shadow. Even the bleached bone finish of the Lizardman temple was build up from a black sprayed base coat - I had tried with a lighter brown textured paint base at first and abandoned this and went back to a black base because the detail just wasn't coming out.
Drybrushing is an easy and very effective paint technique in which the paint on the brush is wiped off on paper or similar until the brush is almost dry, and then this is brushed lightly over the surface of the model so that the paint only catches on the highpoints. This picks up the detail and leaves the original base colour unaffected.
When painting remember that paint effects have a 'scale' too - if you paint very bright black and yellow stripes on an Epic scale model, for example, you 
may feel that it just doesn't look right but not be able to put your finger on the problem. This is because the colours just wouldn't look as bright at 'real-life' scale. Try lightly brushing the colours over with a base colour to tone them down. The black and yellow stripes on the doors of this Epic factory, for example, were lightly brushed over in silver to tone them down.
You also see this effect when making rocks out of broken fragments of real rock - often these to not look right unless the rock fragments are painted, which seems a bit silly (painting real rocks to make them look more real) but the crystal or particle size of natural rock is not always correct for the scale at which you are using it. Rocks take a drybrush technique very well especially if the corners and angles are accentuated with a light drybrush.
I find that natural wood such as balsa also looks better is painted rather than used in a natural colour or stained. The example in this picture has been 
painted in Citadel Bestial Brown and then drybrushed with a light brown - I usually use Snakebite Leather lightened with white.
Metal can be simulated easily in plastic or cardboard and then painted with Chainmail or silver. If using cardboard try pressing it onto some scattered sand to get a pitted effect in the surface. Spray black and then dry-brush with silver to get the effect shown in the tank detail, left. For a galvanised steel effect such as that on the 40K refinery I use an old, large-ish modelling paint brush and stub the bristles on the table so they splay outward like a chimney-sweep's brush. Then stab the paint onto the surface rather than using strokes from the brush so that the splayed bristles give a random criss-cross pattern in the paint finish. Finally use washes watered down 50/50 with water to give a rusty, aged or scorched effect. For rust I use Citadel Chestnut wash. For ageing I use a black wash. For heat discolouration (for example on metal chimneys or exhausts) I use a blue wash and then follow it when dry with a Chestnut wash.
I am often asked how I got the concrete slab effect on the 40K refinery (most clearly visible in a the fans page. This was painted with a thick, sticky coat of grey powder paint and allowed to dry thoroughly. The paint dried with a rough surface. I then dry-brushed with light grey and white. The whole thing was sealed with matt varnish.
Remember that poster and powder paints are not water resistant and must be sealed if you want them to be permanent. Use a spray or brush matt varnish.