Your opinions about KOETSU URUSHI BLUE or VERMILLION

If truly 12g it’s rather light so you would need use a heavier headshell or add weight as has already been stated.

But I have run plenty of arm and cart combos that theory and vociferous members insist would not work.
But they do sometimes so go figure.

However.....
Maybe consider something that is a better match to the tonearm though...

As I recall, all the color-coded variants of the Urushi vary from one another mostly by the lacquer finish.  However, there is one, and it might be the Blue or the Red (Vermilion), that uses a different coil wire (e.g., silver coated copper vs copper) and therefore would stand a chance of sounding different from the rest of the Urushi models.  This info is available on the Koetsu website, such as it is or used to be.  Happy owner of a Wajima variant here.

I believe the Vermillion is unique in that it has half the output at 0.2mV vs. the 0.4mV of other Urushis and the non-Platinum Rosewood - due to its unique coil configuration. If you go for Vermillion I think you would be advised to pair it with a SUT of a high ratio (30x), rather than trying to push enough gain from an active MC stage.

Rather than spitting hairs among Koetsu's I would be casting a wider net and looking at the Hyperion from Soundsmith. As long as you're in that price range. Same or more output, half the moving mass, lifetime stylus replacement, best warranty in the business. What I have my eye on anyway. After the current Koetsu Black Goldline...

Miller.

I just bought a Black Goldline, should get it in this week. Hope it lives up to expectations!