Spirulina did not work for me at all. It's extremely photosensitive and fades in even the least bit of light. What I like to use for all natural green colorant is spinach powder. The color has much more staying ability. Chromium Oxides is my go-to for greens, but if I'm going natural or want a specific look, spinach powder seems to be the way to go.
As for the indigo, I've read that adding the indigo to the lye water is supposed to create a richer color, but personally I prefer to infuse it in a little bit of oil and add it to the soap batter at trace (just as you would with an iron oxide or ultramarine).
I'll post a pic of a soap I did a few months ago that has half spinach powder and half indigo. This was for roughly a 5.5 lb. batch, and I used 2 tsp. of indigo powder to color half of it. So we're looking at 0.75 tsp. or so per pound of soap (not oil), and I'd say that you could probably go up to 1 tsp. per pound of soap or more without the bubbles discoloring.
ETA: This batch is 4 months now, and the color is sticking just fine.