I use modelling chocolate for plaids, but you can use fondant and just paint it with food coloring. I think that I saw a tutorial on youtube from a CC member (Aine?) who was making fondant tartans. Try looking "tartan" up on youtube and see what comes up.
I did one, and tinted the fondant the base color of the tartan (in this case, deep blue) then hand-painted on the stripes using slightly thinned gel food color.
aine's youtube video on making tartans.
http://www.youtube.com/watch?v=6CC4m5neNkE&feature=PlayList&p=81DDD9C9171A19ED&playnext=1&playnext_from=PL&index=92
Oh good grief! I've just watched the video and it is absolutely awful quality and also I don't know why on earth I chose such a dark design in the first place.....and so small!! I made this much too messy and complicated when really all you need is a couple of coloured pens and a white line to make a good effect. I think I'll redo this one for you this week and use much better lighting and a little less fuss! I'm glad to know that the video has been helpful but I can't apologise enough for how bad it looks. I'll replace it totally and let you know when I've done it.
Quote by @%username% on %date%
%body%