I'm a bit curious towards something; do anyone know why facial tattoos are banned in some actual militaries? The most I could find about it pertained to parade uniforms.
I managed to find some more a bit after I posted these comments;
"Conformity in terms of unit cohesion. The military depends and thrives on the establishment and maintenance of a high degree of conformity within their ranks. This is one reason why they issue "uniforms", i.e. - a word that literally means "one shape".
This would depend on the squad make-up, I think, since the aliens onboard might have an exception clause but Ashley's uniform in Mass Effect 3 did not seem to match the standard Alliance uniforms. James Vega was additionally not bared from the N7 program despite having a very visible neck tattoo though that was in the neck and side area and apparently that is considered tolerable in some actual militaries.
Shepard, however, has been allowed to personally disregard uniform in Mass Effect 3 as they were able to wear non-uniform apparel onboard military vessels while on-duty and customize their field armor both in terms of item modification but also color modification. They also have been allowed to wear non-Alliance armor in Mass Effect 1 but I'll have to see whether that can be obtained prior to becoming a Spectre since if it is after, the Spectre occupation might add an exception.
"There are restrictions on the individual occupying special duty positions where they are more likely to be exposed to news media, foreign dignitaries, etc."
Considering how Shepard was allowed to behave around diplomatic personnel and sweet jesus' christmas cake, especially the media, I do not think I would consider this to be an applicable reason.
Note: I am not taking Mass Effect 2 into consideration as neither Shepard nor their squad were considered a part of an official military during those events as far as I can remember.