Very, very rarely were there any type of armour color co-ordinations among a clan/unit's men. Take a look at the paintings for the engagement of Sekigahara and Ōsaka. While clans might colour-co-ordinate their flags and banners, almost no clan does so for armour.

Unfortunately I can't find a high-res link to the Mimikawa painting, but here's a low-res one with the Ōtomo trying to cross the river from the right and them engaging the Shimazu on the left bank. Though not high-res you can still see there's no standard colour scheme on either side.

A possible exception is the team of Shimazu gunners on the bottom left (thanks publisher), who at first glance seem to be co-ordinated blue. But if you look closely it seems they just all happen to all be wearing lamellar curiasses that use blue cords to tie the plates together, and other parts like helmets and leggings aren't co-ordinated. Also the other Shimazu warriors around them, including their officer, isn't bothering with keeping the colour scheme.

So as long as you keep the iron parts black/dark-brown and use one of the common colours used for the cords in those paintings, you can't go wrong. You can even make it fairly colourful, as some samurai evidently did.

For reference, this is the armour the Ōtomo family donated to a temple.

The Ōtomo, like other clans, have very many different crests used on their banners, depending on the branch and the leader. You can see some examples here. Of the two banners on the next page, the one on the left is based on a surviving example. The Ōtomo is said to have added cross to their banner (right), but the claim is unsourced.

Finally, only a minority of men actually wore sashimono, and it seems to be even less common among those fighting on foot compared to those mounted. Which is to be expected.