Here is a pic of what I've done. This is actually code 40 track, but that doesn't matter. First step is to prime the track with a Model Master light gray color from the rattle can, since with out a primer the rest of the paint will have a hard time adhering, and likely will rub off easily during ballasting. Next, I apply washes of burnt umber oil paint, thinned with mineral spirits. After drying overnight, I apply additional washes of black and raw umber, trying to vary the color a bit across ties so that it does not come out too uniformly. You can vary the tones to get the more predominantly black effect that you want.
You can try something similar with acrylics, but I think the oils work better for me since they dissolve more readily and bond better when dry. Also, the subsequent washes will blend better with previous coats, whereas acrylics become impervious after they dry. Acrylics might also dry too quickly during the process.
I do paint the track before installation, since I can do it outdoors or in the garage and avoid the indoor paint fumes. However the paint can make the flextrack somewhat stiffer. For the rails, I used the Woodland scenics paint pens, and I try to apply it just so that it also covers the tieplates/spikes.
I suggest trying it out on some sample track, so you can find the techniques and colors that give you what you are looking for.
HTH,
Ed
