Mixed paint is just taking colors and mixing them on them on the pallet before putting them on the canvas. Layered paint is putting down one color. Letting it dry and then putting another over top of it. This gives your painting more depth in the color as the light reflects through each layer. This is much better seen in person than through video or picture.
I would recommend a combination of mixing and layering. I always layer my colors. Some of the layers are mixed. Some of them are not, but this allows for your painting to not look as flat. If you’ve never tried layering, you should try it out. You may find that you enjoy it. Below is a picture showing the difference between mixed and layered paint.
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