I'll be using the green base color for all the different shades of the panels. I just add a little white, black, gray, or whatever to get a nice variety.
Here, I've added a little bit of pale yellow to my base color, and painted some simple geometric shapes all over the green portion of the ship. I find that having a variety of shapes works best, though you might like to experiment with all-squares, -rectangles, or what have you.
This process should be very fast. You don't need to worry about getting even color for your shapes, because our goal is to get a variety of shades.
A quick compliment for Partha Paints and IWM paints (they are the same thing). I love these paints. I am particularly happy with them because they dry so flat! Lots of paints out there have a satiny or even glossy sheen when dry that I find very annoying.
Now I've added a bit of light gray to my previous mix, and simply painted on some more shapes.
Again, no need for consistency here, consistency is not what we are after at all! Throw shapes wherever you think they look good, and just go with it!
In this picture, you can see that I've added some more light gray to my mix, and painted on still more little polygons. These are a bit smaller than the last ones because there is less room to work with!
An important note here: don't get too carried away with your polygons! You want to make sure you leave the basecoat showing through - it's one more color that you want to leave in there to help you make your little mosaic pattern.
At this point, I'm finished adding panels to the top of the ship. Now I do one last step to add a little more variation to the individual panels - a wash.
I mix up a fairly thin wash of green ink, black paint, and water (no Future or anything to make the wash behave - I want to encourage a little bit of pooling). I apply this was to the entire top portion of the ship. This helps to pick out sculpted panel lines; dirty things up a bit, and make the panels look a bit spotty - just what I want!!