I'm not sure about an ADA act violation, but working in the NICU myself, I can see how being colorblind would put patients' safety at risk.
American with Disabilities Act
I'm so sorry to hear about this, I'm sure you must be devastated having your heart set on NICU. I am not familiar with the ADA act, but it may be a violation, I would recommend reading it carefully! You may also think about contacting someone in your state's HHS department that would know more.
Good luck with everything