Gunfreak, here is a URL listing descriptions of the
colors carried by Iowa troops during the ACW.
Items 16, 17 and 18 are of the 6th Iowa's colors,
both the National Color and the Regimental Color
(called 'Banner' in the listing).
link
The 'Banner' (or Regimental Color) is just the normal
Dark Blue issued to Union Infantry with a central device
of a pale background with the Federal Eagle imposed
as was issued to almost all volunteer units when they
mustered into service in 1861.
Below the central device would be a 'ribbon' outlined
in gold/yellow with '6th Regiment Iowa Volunteer
Infantry' inscribed thereon.
The National Color is the normal US Flag for the period
with about midway down the stripe pattern the words
'6th Iowa Volunteers' (without 'infantry' or 'regiment')
inscribed.
Neither Color had, according to the source, any battle
honors inscribed.
This doesn't get you a set of 28MM Colors, but you could
buy a normal set, copy them and then photoshop
the necessary information, creating your own correct
Colors.
Here also is a URL giving you a short history of the
6th Iowa's participation in the ACW:
link
Probably told you more about aardvarks than you wished
to know…
Hope that helps.