Why use mixed media?

A few questions came up in the comments of my recent YouTube video about painting techniques with watercolor and gouache:

William Trost Richards, Mackerel Cover,  courtesy Smithsonian
Pen and black ink, brush and watercolor, gouache, 
crayon, traces of graphite on heavy gray wove paper

Sujanith Tottempudi  says, "I can't understand why use mixed media....when each medium has specific principles..."

James Gurney says: Maybe it's semantics, but I don't see it as mixed media. Watercolor and gouache are both made of pigment, gum arabic, and water. They're really very similar. By using a variety of techniques (transparent, opaque, wet, drybrush, etc.), you get the same range of effects that you would get with oil paint or any other medium.

You can paint with pure transparent watercolor and save out your whites if you want. It's a very attractive technique, but that's just one way of working. The medium doesn't come with absolute principles about what techniques you can do with it.

Veaudor  says: "Did you say that there's black on your pallette?? Mon dieu! I never had a teacher that ever recommended black. I swoon! ;*) (PS I'm gonna try this...)"

Gurney: Yes, try it out. I don't always use black. I often mix my "blacks" out of a dark warm color and a dark cool color in order to maintain some color character in my blacks. But black as a pigment can be helpful, especially if you need those ultra dark accents. I also love it for doing black and white grisailles. Some teachers discourage it because if students always use black to darken colors, it can deaden the mixtures. But if Sargent, Zorn, and Velazquez used it, I think I will include it in my kit of options.

Oğulcan Yolcu  asks: "Which white color do you use?" 

Gurney: I'm using M. Graham titanium white. I've used zinc white, too, but not as much.

ROBERTO HILARIO LOPEZ GARAY  "How can you paint in such small surface?"
Gurney: I'm inspired by the gouaches of Adolph Menzel and William Trost Richards, which are tighter and often smaller than mine.

ryan de carte  asks:"How do use ultramarine in washes without getting so much sediment and separating from the mixture when it dries? I’m not sure how it is in oil but gouache, watercolor and acrylic have this with ultramarine."

Gurney "I haven't had that issue with sediment in ultramarine. More so with the heavy inorganic pigments like manganese blue or cobalt. That sediment texture can be very desirable. If you want flatness you can get a flat color out of any sedimenting watercolor by adding a little white or using smoother paper.
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Watch my newest YouTube video about painting realistically in gouache and watercolor.