I wanted to return the favor since you took the time to give me such detailed feedback on my game. 😄
I actually think the vibrant colors are one of the strengths of the pack. At first glance the potions are very easy to distinguish from each other, which is exactly what I'd want from inventory items. I wouldn't tone them down globally just because they might look bright on their own — against a dark inventory background that contrast could actually work in your favor.
My bigger concern would be the 32x32 size. Some of the smaller decorative details that look great at full size may disappear or blend together when scaled that far down. At 64x64 I think the painted detail has much more room to survive.
One thing I'd definitely test is whether each potion still has a recognizable silhouette when you temporarily ignore the color. If two items are distinguishable mainly because one is red and another is blue, they may become harder to identify quickly once you have dozens of them in the same inventory. Your 7-tier idea gives you a great opportunity to make the silhouettes themselves communicate progression too.
So personally, I wouldn't make them less vibrant yet. I'd first test a full inventory grid at 64x64 and 32x32 on the actual dark UI and see which details survive.
And now I understand why your feedback about visual hierarchy on Gilded Dominion was so specific 😂 You're dealing with exactly that problem yourself.