Oooh I hope you get there soon! I, too, want to try to get Soot Black for my Chocobo. Been trying to do the hexadecimal theory, but the numbers aren't coming out correctly in relation to an RGB slider. The closest I got was inputting Desert Yellow and coming up with:

+10 red
+8 green
+2 blue

for something that is close to dark gray. So in theory, if this actually works, Black (if it's 000000) should be:

+13 red
+11 green
+5 blue

I don't have the gil for buying food, so I'm just trying to help out with brainstorming. But I would love it if someone tried this and report the findings.