From Longman Dictionary of Contemporary Englishwatercolourwa‧ter‧col‧our British English, watercolor American English /ˈwɔːtəˌkʌlə $ ˈwɔːtərˌkʌlər, ˈwɑː-/ noun 1 
[countable usually plural, uncountable]AVP a type of paint that you mix with water Margaret began experimenting with watercolor.2 [countable]AVP a picture painted using watercolours a watercolour of the castle
[countable usually plural, uncountable]AVP a type of paint that you mix with water Margaret began experimenting with watercolor.2 [countable]AVP a picture painted using watercolours a watercolour of the castleExamples from the Corpus
watercolour• The acrylic is not effective to use in the traditional opaque sense, it must be treated as a watercolour.• Soft haired brushes are mostly used for watercolour or tempera painting, with bristle mainly for oils.• I always use Langton watercolour paper, which I wash first so that no spots appear when laying on the colour.• I have just started using the medium of watercolour, struggling along and producing paintings of a particular subject.• Other watercolour and drawing papers are available.• Stretching watercolour paper How do I stretch watercolour papers so that they remain completely smooth during use? 