Based on a recollection. Names changed, paraphrasing abounds. This ain't how it really went.
Me: Art Supply Shop, how can I help you?
Customer [Let's call them "Green-Required-Event-Eventually-Needs".]: I'm looking for some green paint, like eco-friendly paint.
Me: Like a particular brand?
Green: I don't know a specific brand. Whichever brand is the most green.
Me: Okay, let's see...what are you using it for?
Green: It's for the eco-home show, at the convention centre. We want to have a kids' station, for them to do crafts, so we need whatever's the most green paint you have. Isn't there enviro-friendly paint?
Me: Well, I don't think any of the paint brands we have market themselves that way. Lots of the sketch papers do; hydro-powered or wind-powered production plants --
Green: --Yes. Like that. The wind-powered paint.
Me: Okay. As I say, I can't think of a paint manufacturer who markets themselv--
Green: --Just whichever one is the most GREEN.
Me: Alright. Well, let's start with this. Since it's for children, the most common type of paint would be tempera, sometimes called gouache. It rinses off with water, and it's made from gum arabic, some binder agents, maybe resin, and pigment.
Green: So it's the most eco-friendly?
Me: Well. Yes and no. Okay, just to be a bit on the technical side: All paints are made of a vehicle, and pigments. The pigments used in any kind of paint, or brand of paint are the same pigments. They're what give it the colour. The vehicle is what the paint is carried in. Oils are carried in usually linseed oil from flax, acrylic are in an acrylic polymer and water-colours are in gum arabic, which is water-soluble. The tempera I mentioned is a lot like watercolour, only the colours used are more opaque, and their may be other chemical binding agents in it. I think they used to use egg sometim--
Green: --I don't want other chemical agents or whatever. I want the best enviro-friendly, natural paint. This is for children!
Me: Okay, yeah, I understand that. Here's the thing. The chemicals themselves in that paint are non-toxic, that's one reason it's popular for kids. Okay, so all paints have a vehicle, and pigments. Vehicle-wise, if you want the most "natural" that would actually be oil paints.
Green: But oils are toxic.
Me: Not necessarily. The pigments are usually mixed in linseed oil, which is just oil from flax. Hmm, probably the most natural of the paints, really. If you use solvents to clean up it would be toxic, but there's non-toxic solvents out now. But it's hard to get out of clothes, so for kids maybe not so good, but if "natural" matters, it might be the best bet.
Green: I heard there's fumes from oils, I can't use that.
Me: The fumes are mostly from solvents people use to clean it up. Using oil paint is like leaving any vegetable oil open in the kitchen. Like having a dish of olive oil on the dinner table. I don't really recommend oils for little kids anyway without direct supervision. It's an interesting idea: "green" paint.
Green: Are you sure there isn't anything? I really need to get something.
Me: Well to be technical again, I guess there's a lot to consider. Let's start with the vehicles. Oils are probably the most natural, acrylics probably the most un-enviro-friendly, since I think the acrylic is probably a petroleum derivative. To think about the pigments, some are more natural than others --
Green: Okay. Give me those, in all the primary colours, and a bunch of other colours.
Me: Well, natural and non-toxic aren't the same thing. Flake white has lead in it. Lead is more natural than say, quinacradone which is used in a lot of reds, but quinacradone is non-toxic in typical use. And I can't gather up a whole rainbow of colours. Some are more arguable more earth-friendly, like the browns. They're usually made from clay silica, like from different regions, which is why they're called raw sienna and burnt umber and such. Arguable they're more eco-friendly.
Green: Only the browns?
Me: Probably not only, but now that I think about it, it could be they're worse than the manufactured pigments: how do they get the clay? Do they clear-cut a forest to get at the clay? Gently by the riverbed? I don't know. Clay's non-toxic, unless the dry pigments are breathed in, then it can damage your lungs, which doesn't normally happen when you're painting. Other colours like the madders are from plants, I think the roots. I assume they're greenhouse grown, that'd be efficient, but I don't actually know. And the non-toxic, manufactured pigments may have other waste chemicals from production that aren't good wherever they're disposed of.
Green: Okay. Umm..
Me, barreling onward: I remember speaking with a customer who was vegan who did murals, and loved our in-house student brand. She was worried the carbon black might use charred animal-bone soot. Carbon black always used to. So I called the manufacturer, and asked. They put me on with the chemist, and he explained these days they make it from acetylene, since it they can have more control. The vegan was happy.
Green: ...
Me: And I guess whatever you end up using, you'll have to consider disposal. Most people just rinse brushes in the sink. But you could have the kids rinse them in a basin, let the basin water evaporate, and you'd just have a bunch of pigments at the bottom. Don't breathe that in. You can see, there's different issues to consider for not only the type of paint, but also the individual colours.
Green: Okay. Well.
Me: The best bet, I recommend, is to just use kids' tempera paint, and maybe have them paint in recycled items, like egg cartons or something. I wouldn't advertise the paint as being eco-friendly, though.
Green. Okay. I'll have to check with someone. She said there'd be environmentally-friendly paint, I should just call the store. We just wanted to use whatever's the most green.
Me: Well if your friend knows of a specific brand, I'd love to know about it: call me back! As I say, a lot to consider. Go with something non-toxic and washable for kids, and that would be the best choice in my opinion.
Green. Okay. Thanks, bye.
Me: Let me know how it goes! Thanks.
- - -
Me afterward, thinking: Damn, I read too many science blogs.
- - - - - - - -
I put three pieces in the show, and attempted to sell three others. Admittedly. turnout was slim, and most of the other artists were not there. This could have been because the gallery show was part of a larger conference with a big attending fee, and it wasn't clear anyone could attend the free gallery show - I couldn't figure it out from the website, and asked someone the day-of. Not to grumble overmuch - the people in attendance were interesting and we had a nice evening.
Michelle joined me, and we had a great time, met some interesting people including artist Karyn Wong and her boyfriend Jacob. Karyn's work is pretty fantastic stuff (digital fairies!) so make sure to check it out.
The process of coming up with a visual metaphor to explain to someone a particular scientific concept can be quite effective, not only for your readers, but for you –– the process can help to clarify the concept in your own mind. In addition, a discussion about the limitations of that metaphor can be just as clarifying (and fun!). We are incorporating this idea in our NSF-funded Picturing to Learn program. 
Writing with Light
