of 
At this point, I break out the airbrush again for another gratuitous not-really-necessary step: filters!
Another kit-modelling trick, a filter is essentially a very transparent glaze that's barely perceptible; the idea is that it ties all the various colours you've used together, lessens harsh contrasts, and so on. I'm using a mix of many many parts of Future Floor Acrylic to a drop of Tamiya's Neutral Grey. After a vigorous shake, the mix looks pretty opaque, but I'm sure you can appreciate in a few photos' time that you can't make out the filter itself.
This really is an airbrush-only trick, unfortunately; the filter needs to be applied very thinly, or it'll become obvious and thus obtrusive.
Next, I paint the bases. Each gets a wetbrush of very dark grey (50/50 black to Codex Grey), a dappling of a mid-dark grey (25/75 black to Codex Grey) and a light drybrush of mid-grey (Codex Grey). Lastly, the black sides of the base are repainted to neaten them up.
This is my dappling brush - an old Citadel brush which has had the point viciously cut off. It's also useful for drybrushing, but I get the impression that I'm one of about three people worldwide who prefer a softer brush for drybrushing.
Next, the sensor eyes are all painted in red. Again, stock Citadel colours - Scab Red base with a Blood Red spot in the centre. I'm not making a special effort to highlight them gem-style - again, for reasons of scale...