The answer concerns balance. Artists are constantly balancing opposing ideas. Here are some opposites between which we must select.
- Tonality, is this painting, or passage, a light one? Or dark ?
- Color, is this painting going to be in earth colors or chromatic?
- Transparent or opaque?
- Tightly rendered or loose?
- High contrast, or low?
- Soft edges or hard?
- Decorative or naturalistic?
- Highly colored or grave?
- Will it have an impasto surface or be enamel smooth?
- Will it have brushwork or no?
The answers to these questions have implications on our methods and materials too. If for instance I want to paint highly colored bright impressionist pictures I would probably use a palette containing cadmiums and a pthalo blue. Is a teacher who recommends that I use earth colors wrong? No, but his palette is not going to help me do what I want to do.
I might want to paint roughly textured landscapes the size of a bedsheet in one shot. I probably wouldn't want to use red sable brushes and real amber medium. Sables and amber mediums aren't wrong, they are just not well suited to the purpose I intend. I would probably want to use big hogs hair brushes and an alkyd medium.
So it is good to weigh a teachers advice against what you are trying to do. When in a class, do it the teachers way and try out what they are suggesting, because that's the best way to learn from them. But when you go home, choose from their method that which suits you. But don't throw the rest away, the art you are making will evolve and you may need it later. Its very handy to know a lot of different ways to do things in painting. You may not need a particular technology today, but you might tomorrow.