As a step mum - I've tried many tried "solutions" in the past.
The fact is that - you get rid of the nits - you get rid of the nits - but they can come back...
As for the colour issue - I know full well about the one up man ship that comes with blended familes. - I think this is the real issue.
Any time I tried to do something for my step kids - there was always a "one upmanship" that occurred as well.
I got annoyed - sure, but also realised this was an issue with the adults rather than the children.
You try to do the best for the children , but have to accept that the person on the other end isn't necessarily acting in the best interests of the children and have to eat humble pie accordingly,
this is not about you or the other parent everthing is about the children...
Much as it hurts - you have to swallow it and figure - whatever happens at the other house happens and go with it.
if the school doesn't accept it - so be it - it's not your issue.
It has taken me many years to get to this point - so I'm posting in hindsight really.