Yep I have them too, Blue is the way I went with them. If you are meaning the raised seam looking thing running around the point where the shoulder/Sleeve would be I would paint it blue.
Elizabethan minis work Jacks like this and often the piping (which is what I think it is) could be coloured or it could represent it being a sleeveless jack. But these figures are for 1700 onwards so would go for blue
PS, your name "15mm" may throw people when looking at the thread which is about 28mm