Low Key

formal —> low key color spans the range from mid-tone to black.

Source —> http://www.sensationalcolor.com/understanding-color/theory/high-key-low-key-color#.WOO7FTz3aEc

informal —> low key scheme has dark value of color

High key

formal —>High key color describes the set of colors that range from mid-tone hues to white.

Source —> http://www.sensationalcolor.com/understanding-color/theory/high-key-low-key-color#.WOO7FTz3aEc

informal —> high key scheme has light value of color.

Narrow range

formal —> less difference in color values

source —>http://www.ldoceonline.com/dictionary/narrow

informal —> when values are congregate

Broad range

formal —> big difference in color values

source —> http://www.ldoceonline.com/dictionary/narrow

informal —>when values are spreaded

Texture density 

formal —> Texture gradient is the distortion in size which closer objects have compared to objects farther away.

source —> http://www.sciencedirect.com/science/article/pii/S0042698901001213

informal —> the objects those are similar

Gouache

formal —>a method of painting using opaque pigments ground in water and thickened with a gluelike substance.

source —> http://www.dickblick.com/categories/gouache/

informal —>solid colors