When painting things that are of a mechanical-industrial nature it helps to remember what it is that you are dealing with. Metal and cement cover 99% of the mechanical and industrial things you will paint. To get a mind set going, I will start by saying that most metal and cement are grey. Different types of metals under different conditions will have many varied colors. When heated, some types of metals will literally produce the entire spectrum. Cement also appears in a wide range of coloration, from a near white, light gray, through darker (and browner) aged concrete to a moldy, rain and soot streaked combination of gray, brown and black. Usually they have oil, rust, dirt, and dust on or around them. LIMIT your colors. Black, white, brown, burnt sienna, dark blue, yellow ochre, chrome. No bright colors unless you want to have some toxic ooze which is really bright or perhaps lettering/ hazmat symbols on the models. The dull colors will make the bright ones look REALLY bright.
After priming with Krylon Sandable Gray, apply a thinned coat of black using a stiff bristled brush,. Use enough water so that the paint STARTS to run. Not so thin that it all runs off. Have an old rag handy for the next step. Immediately after painting on the black, grab the piece in your hand. Close your hand around it, switch to your clean hand and repeat. Dry your hands off and repeat grabbing. If possible roll the piece in-between your hands like you are warming your hands up. Keep doing this till not much paint comes off on your hands. Let it dry (and wash your hands) :-)
Now get a 1/2" to 3/4" flat brush with moderately stiff bristles. Put a small amount of very light grey paint on the brush and rub it around on a scrap piece of plastic. This is to get the paint into the interior bristles of the brush. Now take a piece of paper and paint back and forth till the paint on the paper looks more like a light smudge than PAINT. What this step is for is to basically run the brush ALMOST out of paint. Once you are down to getting just a light dusting of paint then scrub away on the model. Being that the brush is almost out of paint, you will need to repeat the steps above a few times to cover the model. While dry brushing, try to constantly rotate the model so your brush strokes are not all going in one direction. Watch the smaller details of the model as you drybrush. You will notice that the details become more pronounced as you keep brushing over them. After a point they will begin to be less defined as more and more paint goes on. You will just need to play with this technique to see what the feel is and what the limits are. When you get the brush to the right amount of paint (by almost running it out of paint) the effect will be subtle and smooth. The details will seem to almost magically appear without being overstated.
Now that you have things highlighted it's time to add color. What will make a piece interesting is variety. To start with, I'll keep it simple. Use blue and brown. Dilute them WAY down, to a thin wash. Select a part on the piece, a pipe, a metal plate, a vent etc.. and put a wash over that piece only. Do a few other parts this way. Switch colors and put a wash over some other parts. The amount of parts you color which color is completely up to you. By keeping the wash very thin you should end up with tints of grey on the model and not a BLUE pipe or a BROWN fitting. Another interesting thing to do is to put a thin wash of one color over another and end up with a third color.