Where we have a items of one type, b items of another type, c items of another, etc., we can solve for the number of permutations with the equation:
$$\frac{\left(a+b+c\ ...\right)!}{a!\cdot b!\cdot c!\ ...}$$
We have 13 total stripes. There are 6 of one kind (blue) and 7 of another kind (red). This gives us: $$\frac{(6+7)!}{6!7!}$$ $$\frac{13!}{6!7!}$$ $$\frac{13\cdot12\cdot11\cdot10\cdot9\cdot8}{6\cdot5\cdot4\cdot3\cdot2\cdot1}$$ $$\frac{13\cdot11\cdot9\cdot8}{6}$$ $$1716$$