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Green comes from chlorophyll, which is held in place in the plant by chlorophyll water soluble binding proteins (WSCP). These proteins are soluble in water, mildly soluble in alcohol, and insoluble in non polar solvents.
Butane is non polar, but will still absorb up to 32 ml/liter water, which can break down these proteins and expose the chlorophyll to be transported away in the form of micelle.
Ostensibly, the ability of a non polar solvent to transport these micelle, is proportional to the amount of water that it has picked up, as the water is doing the transporting.
I sometimes pick up a light electric green hue extracting with butane, but never a dark green. The amount of green appears to be influenced by the water content of the starting material, temperature, and soak time, as well as how many leaf margins are broken, so as to expose the chlorophyll binding proteins to water. Finely ground material exposes more margins, and extracts more chlorophyll.
PS: Filtration can influence green pick up too, from suspended particles.
you are right that butane is non-polar. But the butane you are using to do extracts isn't pure butane. depending on brand it can have anything from propane to isopropyl mixed with it.