I have always had trouble mixing colours; more so now that I’ve switched to acrylics and so many of the pigments are new. Everyone knows the standard blue plus yellow equals green. It’s the more difficult, Burnt Umber plus Mars Black plus Naphthol Crimson plus Hansa Yellow, equals a great colour for tree bark, which makes life difficult. Some of these new colours may even be unfamiliar to some artists; they certainly were to me.
Some of the traditional pigments are poisonous, or at least bad in larger quantities. I wouldn’t ingest any of the newer ones either but most are not actively poisonous and many of them use better pigments than the old standards. Cadmium is a good example.
Cadmium isn’t overtly poisonous but it can certainly poison you if too much is absorbed or ingested; so I no longer use Cadmium Yellow or Red. The newer pigments seem as good or better and they don’t have the health concerns. At least not that we know of.
I have used; Prussian Blue, Chrome Yellow; Cobalt Yellow; Naples Yellow; Strontium Yellow and Vermillion. I still use Burnt and Raw Umber. Most of these were used in oils but many exist in acrylics. The benefit is that Acrylics use more of the newer less toxic pigments. I’ve also read that the acrylic medium is more stable over longer periods so there should be less chance of toxins escaping.
It’s difficult to completely stay away from poisons when painting. I watch children carefully if they get near my paint supplies. I assume adults are not going to crack open a tube and take a lick. I’m always a little concerned over components that can be absorbed through the skin; like Lead. Lead white is still available but Titanium White is better and much less hazardous. Curiously I don’t think Lead can actually be easily absorbed but the chemicals containing the lead can. Cadmium is similar. Beware those of you who like to paint with your fingers.
For a more complete list of poisonous compounds used in paints go to: http://captainpackrat.com/furry/toxicity.htm
There is no guarantee that all of the modern pigments are completely safe either, so pay attention.
Getting back to the original subject; I mix until it looks right. A problem I had years ago was trying to use too many colours. Now I try to limit my colour mixing to two. As soon as I try to use more colours it turns out brown or grey. Of course occasionally that is exactly what I am looking for. Sticking to two colours isn’t always possible and it often leads to more/different paints. I tend to use Ultramarine blue but sometimes Cobalt blue is the perfect sky colour; Cobalt is bad so Phthalo Blue is a good substitute. Hansa Yellow is very transparent so I often add it just to warm the colour. Quinacridone Crimson is absolutely beautiful. Purple has always been a muddy looking colour to me so Dioxazine Purple is a welcome addition; I’ve used it on grapes. These paints are often quite transparent so I use Titanium White underneath; wait until it dries then paint on top.
I mix colours in pairs until I get what I want. It may need to be a little greyer or darker so I used to mix in Pains Grey; Mars Black is a good substitute although a little warmer. If the colour is lighter then I use Glazing Medium. This is where underpainting white is useful. If the colour I’m trying to paint isn’t very pure then adding white is fine. However if it’s a pure colour then white under colour plus glazing usually works.
There are new pigments in the works. I’ll wait a while to see how they turn out but I’ll definitely try them when they are available.