Potters & Sculptors - Making Rock from Mud
Light rutile is dark rutile which is already fired to eliminate off-gassing from components which are not rutile, so you want light rutile. - https://digitalfire.com/4sight/material/rutile_1204.html
Rutile, being a natural ore, is one of the most frustrating ceramic ingredients I've used. Every time we buy a new 10 pound bag of rutile, the rutile glaze looks quite different from every bag of rutile we've used previously - which can be good or bad.
I recently bought some titanium from a different supplier. I swear I got rutile instead. I won't be buying titanium there again. I find it to be incredibly frustrating as well. Every time I try a different supplier for titanium it's the same issue as you describe. Completely DIFFERENT glaze results, be it color or texture or anything.
I immediately labeled the jar of 5# "bad titanium maybe rutile." I then ordered 5# from my online supplier who has the better stuff.
BLEH! I don't like rutile at all. I found it's way to unreliable. I much prefer titanium, and even it has problems.
I love rutile! If the recipe just says rutile, I've used regular rutile; it hasn't presented a problem so far. The studio where I work has both, but at home, I plan to run some through a bisque fire if I need light rutile.