Landscape is one of those nouns that is dual-mode. It can be used as a count noun (most often) and as a noncount noun. In speaking about landscape painting (ie landscapes, in general) the following are possible:
as count nouns
Use dull colours to depict a gloomy landscape.
Use dull colours to depict gloomy landscapes.
as noncount nouns
Use dull colours to depict gloomy landscape.
However, I doubt grammar-checkers can cope with this variety.