You sure it's a N-445 and not case file review letter?
If it is indeed N-445 oath letter, the DO may have inadvertently copied/printed it in yellow.
I am not sure, because the office told me that the ceremony here in Austin was going to be
on may 20th but it has changed to may 25th.
I got one form N-445 ten days ago and it is yellow.
it is yellow for everyone?