I think the color will be similar, but you said B30 in one and B25 in the other. So I think the size difference makes them different and is a reason for having both (assuming you find uses for both sizes).
I think different due to size and color. I have a BN Kelly 28 and noir Birkin 30, both PHW. I use for totally different wardrobe color ways more so than the bag style. I can't do a pic right now but can later today. Let me know if you think that will help.
I also have a blue nuit K28 GHW and a black B30 GHW, I wanted to have both blue nuit and black so it doesn't matter what others think. If you are hankering for black, get it.
I wear a lot of black and a lot of navy. I don’t find them similar personally but I prefer neutrals. I have attached a photo of both in Clemence. Both get a lot of use and I have a blue nuit k with phw on my wish list (have a black one). My black H bags have ghw and my blue nuit have palladium.
I have a birkin 30 in both blue nuit PHW and also a birkin 30 black PHW. I use them with totally different outfits and never considered not having both. That said, I really only have neutral bags in my collection and wear mostly neutral outfits.