(if you want to know what this method gives at the end, see my first drawing: the Pininfarina Mythos:it was made with the same method)
I just want to advise you before that I am a french speaking person so...
I added color.I prefer red because I realised gives more punch and it's easier to use than other colors. The color of my drawing is not necessarly the same as the photo,like you can see. My basic method is to put color everywhere and after add more or less black(for shades). If I don't press strong on the coal, we can see the color through: it gives a dark red in this case. My drawings are never exactly like the photo: I modify it as I want.For example, the rear end of my Mercedes is shorter than the photo. When you change things from the photo, it's not always bad:sometimes it's better. I don't like to copy exactly what I see. You can see also that the color don't ends at the lines or it's not precise: it dosen't matter because I will add details after. Anyway, that's also my style to not be precise.I put color everywhere on the body, even at the places I will add black.
For the color,I use ink from Crayola markers: the ones for children.I take off the cartridge and I use it as a paint brush. The cartridge is a sponge soak into washable and non-toxic ink. All I have to do is soak the cartridge into water and the ink becomes active. Because there is lot of water, the paper becomes rinkle but this dosen't matter for me.I tried with water paint but it dosen't gives someting as nice as the ink.
The stage 4 is the addition of coal and basic shades.It will comes soon.