Well on my Tamiya Char B1 I used Silly Putty for the masking and then drew the black demarcation lines in with a Sharpie brand felt-tip marker. I was able to cover the few areas of paint creep under the marks with the Sharpie as well. Afterward a few filters toned down the contrast. If you can't get Silly Putty or a Sharpie I'm sure you could substitute other items. 

Silly-Putty

 Putty removed:

Filters and mud added:

I agree with Gamera that silly putty and a sharpie is the way to go.  I used the same method as he has explained on my Somua.

Good luck with your build and put up some pictures of your progress.

Where do I get this silly putty because I tried to get this product in the past and inside the box the substance looked completely different.

Howdy!

You can also use other products instead of silly putty - like for example Pritt Sticky Tac, UHU Patafix or Faber-Castell Tack-it - or other similar. Those are sticky masses that can later be pulled off without leaving a residue. Hope it helps, have a nice day

Paweł

Not all French tanks' camo had a hand-painted demarcation line.  Footage exists of factory workers spray painting the black/dark brown lines on Renault turrets.  Photos of various vehicles confirm this.  Check your photos.  When I did my Char B1 bis, I sprayed the hull's demarcation lines with my airbrush set at its finest setting.  For the turret, I just used a narrow brush.

Yeah, when I started the Char B1 I was surprised at how many different camo schemes the French had. Something like five different types on the Char B1 depending on what factory turned it out. I'd assume the Somua would be very similar. I'd never seen one like Rob's until he posted photos.

And poster tack type putty should work almost as well as the SP, just a little less stretchy.