Sentence:  Often , in working out-of-doors under all conditions of light and atmosphere , a particular passage that looked favorable in relation to the subject will <WSD>be</WSD> too bright , too dull , or too light , or too dark when viewed indoors in a mat . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)