Well, it turns out I didn't like the colour enough, so I have started to repaint them.
I used Tamiya Gun Metal and Tamiya Black, mixing them on the brush - a cheap brush. I want the track plates to look slightly different colours - some darker, some lighter - so I didn't mix up a big batch of paint. I want the non-uniformity.
After these have dried they will be getting several washes with different brown mud colours. And when those have dried I will see how I feel about them; maybe add some mud, maybe add a lot of mud!