Best natural green food dye

I ended up finding Chlorella algae at a local health food store in powder form. Dusting the curd before pressing worked out perfectly. I held back about 1/3 of the curd so that some marbling would be apparent from the outside. I can't wait to cut this open in a few months!

If you have access to dried sage, or even can dry your own, dusting the curd with ground powder will give a much stronger color (and flavor) than sage juice - a light and silvery green, but something. This can be supplemented with any other green herb or leaf (powdered) whose flavor and color complements, and coated thickly for stronger marbling effect - the moisture in the curd will hydrate the powder so the texture is smooth, but the paste will be concentrated around the edges of the curd pieces much more so than a juice, and marble very clearly.

Otherwise, the juices Dorothy mentioned, spinach, spirulina, algae, seaweed, wheatgrass, should all color green fairly well. If the color isn't to your liking, you might try a more concentrated version of the dye - maybe reducing the juice until quite dark and thick. Most simple juices are rather dilute, so they can't color even as strongly as they can if concentrated.

Powder will, again, likely give a stronger color than just soaking in juice (which is, by nature, diluted), but also a stronger flavor - not necessarily a negative, but something to keep in mind. I have seen powdered chlorella algae and powdered spinach sold specifically as natural green food coloring (dark and light, respectively), so they can certainly be used for this purpose... and the recipes' pictures include sweets, not just savories, so the flavor might perhaps not be too overwhelming if used carefully even with a bright color.