Ok, from an Etiquette perspective…
It is true, it isn’t kosher for a Host to dictate specifics on a Dress Code ** (which is WHY you aren’t supposed to write BLACK TIE on a Wedding Invite)…
BUT for practical purposes, and estetics it is something that is still done fequently (as Bees have said often when dealing with their Wedding Invites … some people need specfic info… otherwise they come to a formal-ish Wedding in Shorts, Ts and Ballcaps). Unfortunate, but the truth. Which is precisely WHY you see things written out on Invites / Wedding Websites etc… REQUESTED ATTIRE is something that is common place nowadays, because sadly, even grown-ups don’t seem to be “socially mature” enough to realize for themselves what is and isn’t appropriate any more.
That said, we are not talking about a Wedding here… we are talking about a Birthday Party.
To be exact, an 85th BIRTHDAY PARTY… this is a BIG Celebration of life… and quite an accomplishment (I always say anything over 80 is pure gravey)… and quite honestly as someone who is over 50, I must say sadly, that from my age forward, a lot of great people pass-on way too early (I myself having lost perhaps 10 or so friends in the last 5 years).
So at 85 your Dear Uncle has probably seen more than his share of that in the last 30+ Years.
Which from his mindset, makes this most definitely a time to Celebrate for sure.
I’m guessing he’s specifying NO BLACK, because it is a colour associated with Funerals… and the last thing he wants for his 85th is to be reminded that the Grim Reaper could be lurking in the shadows on such a joyous day.**
It is a time for Celebration… and wearing colour is one way that folks express joy… so my advice, even though I am sure that the black-trimmed dress is lovely… it is time to BREAK OUT THE RAINBOW… and leave any hint of black behind
** NOTE (linked to 2 references in the above reply) – When it comes to social graces, sometimes AGE can get one a BY from time-to-time… and this is exactly one of those occasions. So although it isn’t appropriate to push for a dress code with one’s Guests, it certainly is understood WHAT the reasoning might be. I’d therefore opt to RESPECT the Uncle’s Wishes… and make a point of wearing NO BLACK whatsoever.
Hope this helps,