Whatever you do, keep in mind what the models should be used as.
Models used for display and pictures can have really elaborate and effective "real" camo. It shows up on the pictures and is really enjoyable to watch and appreciate.
On the other hand, models used on the table in conjunction with a lot of other identically painted models need to have more of a "Hollywood" camo.
They need to have the illusion of camo without the effect of camo. A few groups of models with realistic camo just looks like blobs of colour when viewed from the typical tabletop distance.
The point of painted vehicle camo is to break up the outline of the vehicle thus making it harder to identify, and even making it hard to recognize as being a vehicle in the first place.
While this is very useful in real life it is, in my experience, not so good on models used for gaming.
Another example is face paint. This is most often demonstrated in Hollywood movies.
The point of facial camo is the same as for the vehicles. The breaking of outlines and making the face unrecognisable as being a face. This is mostly done by inverting the way light and shadows fall on faces.
This means painting black on the light areas (such as nose, cheek bones, eyebrows, chin and ears) and green on dark areas (such as eyes, under the nose, cheek hollows). This can be further broken up by the use of brown.
Here is where Hollywood steps in. If Hollywood used real face paint, we would have a hard time recognizing the individual actors. So they created the illusion of camo without the effect of camo.
This is Hollywood at its "finest".
This is more realistic.
The colours on the second "realistic" picture might be slightly odd (and
CG), but the application is pretty good. And of course, hail to the ultimate badass.
So, how do we actually achieve this?
Well, its mostly about allowing contrasting colours to play along and how less is more.
Contrasting colours, such as differently coloured shoulderpads (an iconic Space Marine feature), prominent chest eagles (what Space Marine would hide the symbol of the Imperium?) and distinctly coloured weapons (good for
WYSIWYG) is one way.
Keeping in mind that less is more, is another.
See how the realistic camo is pretty busy and "jumbled up"?
GWs suggestions, while oddly coloured, has are more usable intensity that is seen as camo while not actually being it.
Notice how the Guardsmen have contrasting armour, belts weapons and details? That is part of what makes them recognizable as Guardsmen, even while wearing camo.
The colour choices might not be to your liking, but that isn't my point. The "amount" of camo is though.
Practically I find it easiest to use a triad of colours. Use the mid-tone as a base colour, add markings with the darkest colour and then add small dots or stripes with the lightest colour (making sure to touch or slightly overlap the darkest tone).
One piece of advise is to make sure the colours are somewhat faded. This can be achieved by a thin wash over the camo'ed area after it is painted, or by lightly drybrushing the base colour over the darkest tone before adding the light-tone detailing.
Wow, that turned into a rather longer post than I expected.
Hope it helps.
"And they shall know no fear" 
Imperial Guard-3537pt.
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3.000
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