I painted some French and Indian war highanders and just did the tartan in green. Looked fine, probably because the red lines in it would be lost at any distance, since they don't contrast very much. Something like the red and white of socks needs to be painted im my view, because they are high contrast.
Crow T. Robot; I like your style!
Clibinarium, you are a far better painter than me but youm are probably wrong about red and green being of low contrast. These two colours are on exactly oppostite side of the spectrum and thus they are extremally contrast colours. It is the brightness of white that makes us believe it is more contrast to red. Arent really bright red and bright green more of a contrast?
Tell me if I am wrong, but this is what I have learned before.
Nevertheless, the best way to paint tartan in 10 mm scale is probably the thing that Pawel made. Bright green points on dark green - brilliant idea Pawel.