I'm guessing you don't know which clone of I. germanica it is though?
That seems to be a common thing. I only know for sure what three of mine
are (assuming they came with the correct names), they are 'Alba', 'Kochii'
(which hasn't flowered yet), and 'Crimson King'. I also have the
'Florentina' and the 'Albicans' worked out now.
I'm now thinking the yellow might be 'Golden Bow', but haven't seen a clear
enough photo of that one to be sure. I'm betting it's an old Sass
hybrid. Pink is another color I haven't heard of.
Sass used some of the same parents that I suspect I. germanica originated
from, so some of his could actually be real I. germanica, just a lot
newer. Don't know if he had any pink IB's. [I thought both were
supposed to be "impossible".] Somebody needs to work out for certain what
the parents of the original germanicas really were though.
If I. lutescens was indeed the small parent, then I see no reason why other
colors couldn't exist. After all, I. lutescens comes in lots of colors,
and if those other colors were to be crossed with a pale or white I. trojana (or
mesopotamica /cypriana /belouinii /amas /etc.; all the same wild species?),
seems to me you could have lots to choose from. I suspect that blue/purple
is most common because the tetraploid parents were this color, and that ups the
odds of bees doing some funny business with any late opening purple I. lutescens
(or whatever) flowers, and lowers the odds of crossing with yellows (bees tend
to stick with one color at a time). The white is the most common mutation
from the usual blue/purples, so it's not surprising that a few exist. Some
of the tetraploids being grown were likely white, and some of the wild I.
lutscens would likely have been white too. Besides I'm sure people back
then looked for the "odd balls" to grow just the way we do.
Well enough speculation.
Does the one in your photo have a stem that tends to curve or go sideways
sometimes. I have one with a similar flower that does, and have wondered
if it's not 'Undulata' (think that was the name I saw).