It is described in the story as:

And it was lovely, too: silk of a blue whose secret the dyers hereabouts would slay themselves to learn. Set on it, in gold and silver thread, were the sun and moon our army reverenced.

and later:

The sun and moon shone in gold and silver the moon on the banner gleamed with a soft silver glow [of its color:] it was as though a section of the bright noon sky had come to rest there

The flag as described bears little resemblance to the real deal, as described here

One quote from the Shah-nameh is very similar to the banner in the story:

azure like the sea, a flag flies, bearing the sun image on a violet background and it is crowned by a moon

Eugene Ipavec, 18 April 2005