What does 'semi veg' next to the color name mean?

The term 'semi veg' does not refer to a color, but is a notation indicating how the leather was tanned. MARGESHERWOOD explains semi-vegetable tanned cowhide as leather processed by mixing two processes: vegetable tanning and chrome tanning^[1].

The two processes are quite different in character. Vegetable tanning is a method of tanning over a long period using plant tannins obtained from tree bark and other sources, so the structure is generally firm, holds its shape well, and undergoes a change where the color deepens as it is used. Chrome tanning is a method of tanning in a short time using chrome compounds, making it generally softer, less sensitive to moisture, and easier to produce uniform color. Since semi-vegetable is the name for processing that uses both together, it should be understood differently from leather that possesses the properties of only one side.

However, because results vary depending on the ratio in which the two processes were mixed, it is generally safer to treat it in accordance with leather containing vegetable tannins. Water spots may remain if it gets wet, so when it gets wet, it is best to pat off the moisture with a dry cloth, dry it in the shade, and avoid direct sunlight and heat. It is also good to keep in mind that light-colored leather may experience color transfer if it stays in contact with clothing that can bleed color, such as jeans, for a prolonged time.