I think the only thing that could be done to improve it would be to add the darker hull red you've identified, but you've already said that's on the way.
And how they usually say? ask and yous hall recieve?
Anyway, it has been a bit messy to get here with the proper color palette, but IJN ships used a "65% brown, 20% red, 10% black and 5% white" formula for their underwater hulls. So If I've not screwed up massively (and I'm quite confident I did not, not 100% sure but still confident) here is Fuso as of 1944 again, but with her proper underwater hull colours:
If it gets the "approved" mark I'll update all the other units too.