The photo shows two details from paintings both of trees (from my Heat and Quiver series respectively). Besides the colors, there is one significant difference between them, the style in which they were painted.
The tree on the left is painted in a blended style, where the brushmarks are eliminated or hidden, and gradations of tone are used to create the illusion of form (3D). This is achieved through blending of colors while they're still wet, and by building up colors and tone using glazes.
The tree on the right is painted in an expressive or painterly style, embracing the marks made by the paint brush and painting knife rather than trying to hide them. While there is still a variation in tone to suggest shadow on one side of the tree trunk, the tones are not graded carefully from dark to light as the trunk curves.
Some people regard an expressive or painterly style to be less finished, or even unfinished. But it's not a style of painting where the end result is intended to look smooth and glossy like a photograph. It's a style which celebrates and shows off the materials made to create it: paint and a brush. The result is something only a painter could produce.
