I'm wondering how much Truecolor Dye one would need to color an entire cloak to be worn by a medium creature. The item description says that one vial is enough to coat a square foot of material - but that sounds like you're using it as paint, rather than as dye, which isn't really the same thing. (Nor do we know the standard square footage of a cloak in pathfinder, or whether we would have to do both sides or whether it would soak through).
I'm pretty sure there is no RAW on the issue of dying clothing (though I'd love to see it if there is) - otherwise, how would you rule it if you were running a game with a PC who wanted to do this?
The 1 sq foot is using it as it's intended, diluting it would mean it is ineffective.
And I'd ballpark a cloak at 6'x6' so 36 sq feet, all cloaks will differ, and in reality there is probably more material on most cloaks but I reckon it's close enough for most purposes, and I'd say you only need to count 1 side.
The idea of "coating" something with dye hurts my brain, but that would still only put the cost at 1800 GP, which isn't bad.