Here's what I do....
First off I start with the black outline. I use a thin tipped modeling brush and don't start right at the edges where I want the line, I start a few millimeters away and work closer to the edge making the outer edge of the line as straight as I can. Don't worry about the line being thick, you go over it with the grey when it's dried.
Once the black has dried, start adding the grey in the same way by filling in the area and working outwards. By doing it this way you are in control over how thick you want the black line. You can make the line thick and even, and once happy with it, add more grey until it is a nice thin and even border all the way around.
The best bet is to try this method on some scrap plastic first. Try is on a pop/soda bottle or something that isn't flat to help get the feel of the helmets curves.
Hope that helps Steve