Thanks all
The camouflage smoking rings weren't too difficult as I was thinking at first. I used Tamiya XF-13 thinned with Tamiya paint retarder (1:1) and applied with a fine pointed brush.
I firstly drawn how I wanted the rings formats based on my references photos using watercolour pencils and then I brush painted over it. Using the watercolour pencils allowed me to correct any shape which might be wrong (just smudge it using a cotton bud moistured in water), before committing with the paint brush.
Cheers