Dyeing white putty not-white?

[Tired]

I have some zoanthids that I want to try to contain to some extent on my rockwork. I've had some success with placing flat shells in their path, so they grow onto it and I can then simply trim them loose and have the shell and contents as a frag. Thing is, my hermits like to flip over shells to look under.
Well, hermits can't flip white putty that's attached to the rocks! But it doesn't look very good.
Is there anything I can use to dye the putty another color? It's the two-part kind that you mix by hand. I know there are purple putties, but they're an even worse color- at least white occurs in nature sometimes.
I have an expired jar of spirulina powder, would it be worth seeing if the green from mixing a bit of that in would take? I think ideally I'd be aiming for a sort of dull green-gray, something unremarkable that blends into the color that rock gets when it's maturing but doesn't have coraline yet.

 

[garbled]

I know they also make a dark gray putty, not sure who, maybe Tunze? In theory spriulina powder would be safe, but I'm not sure how much on the surface would break down into organics. I doubt it would be a nutrient leech of any concern, but I'm not sure how it would maintain color.

I don't think it could hurt anything, at least in small quantity...

 

[TriggerFinger]

That’s an interesting idea, can you mix up a small batch with the spinrulina powder, keep it in a little bit of tank water and see if it holds? I’m wondering if it would keep its shape with extra material added.

 

[Tired]

It doesn't like to stick to itself if you add a load of powder in, and the color is iffy. Putting the powder on the surface sticks fine, but then it's a weird blue-green.

Activated charcoal works much better. I put some in a bag and whacked it with a hammer until it was a powder, then applied it to the outside of the putty. Mixing it in takes much more charcoal to achieve any sort of tinting effect, but pressing it hard to the outside can turn the putty a dark (if slightly uneven) gray. This is definitely worth some experimentation. Still stands out in a reef, but less than the white. And there's no concerns about it breaking down or anything. I did rinse the charcoal-ed putty before putting it in the tank, in case any dust was loose. Also accidentally dumped the charcoal dust on an encrusting montipora while trying to color some putty around it (I was enlarging the frag disc), but I rinsed it really thoroughly and the monti seems fine now.