The hardest part of this project is mixing paint colors accurately. One of the largest obstacles is that not everyone can see the same range of color. Some people are color blind in the classically understood sense, but some just have a limited range of color visibility.
Throughout our time working on this project, many students have been able to identify specifically their own limitations when it comes to colors they can or can't see. I will suggest a square for them to complete next and they'll tell me, "I probably shouldn't do that one because I can't see green very well." Luckily, seeing color accurately is only one part of the painting process, and students who can't see various colors very well can still create pieces that are beautiful and useful.
These two resources are both useful and fun for learning about color and testing color abilities:
The students are also getting better at estimating paint volumes. Mixing paint colors can sometimes cause the puddle to get away from you. As you add a little of this and a little of that, the puddle of paint becomes an unwieldy quantity, and still isn't quite right.