When you're designing tartan, remember to stand back about 20' from a to-scale version on your computer monitor. That way, you can see how the colors combine to give an overall color from a distance. It also allows you to see what elements vanish at a distance.

I'm prompted to say this because, any time you have red elements next to blue elements, the overall impression can be purple at a distance (depending on the relative sizes of the elements). There's nothing wrong with that, and there are several tartans that are spectacularly beautiful because of this effect. But you don't want to be surprised by it!