The goal of the class was trying to concern ourselves with pigment and paint in a deeper way by pushing a small number of colors to do as much as possible.
To work along with my students, I did a painting and mixing grid for Burnt Umber (Blick) and Indian Yellow (Liquitex Heavy Body) acrylic paint. Working somewhat backwards (it's more ideal to do the mixing grid first), I first made a painting (using a black & white photograph for reference) using just these 2-colors:
I was rather surprised to find green tones appearing, especially in the upper right. (BTW, I tweaked the photos to look as they do in real life as much as possible. Some subtleties and colors will inevitably be off.)
I wasn't expecting any greens from 2 warm colors like these. Possibly if I had used raw umber (a cooler color), then maybe yes.
So to prove that I wasn't seeing things, I then made a matrix of possible mixtures of burnt umber and indian yellow and tinted them:
If you look at the center square and some of the ones around it, sure enough, it appears greenish.
And because I had originally started using Indian Yellow in oils, I made this comparison between the 2 versions of Indian Yellow I own in oil and acrylic:
It may be hard to see here, but in person, the Williamsburg oil version (left column) maintains a richer color as it gets tinted. Out of the tube it is more orangey and Liquitex is more like yellow ochre.
The real point here is not so much why unexpected results can occur with pigments, but that you should explore and get to know what your pigments can do!
Pigment codes only tell part of the story, but it is important to be aware of them.
The acrylic colors I used were:
Blick Raw Umber PBr7
Liquitex Indian Yellow PY139
And my oil versions:
Williamsburg Burnt Umber PBr7
Williamsburg Indian Yellow PY83 (Diarylide Yellow)
Given that the Williamsburg Indian Yellow uses a different pigment, you would expect a slightly different result. (BTW, originally Indian Yellow supposedly came from feeding cows a diet of nothing but mango leaves and then collecting and drying their urine to get the pigment.)
Well, at least Burnt Umber is consistent. Or is it? I also have a tube of Michael Harding Burnt Umber which I haven't used yet, and it lists the pigment as PBr6. I would think that a common earth tone like that would be consistent.